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Why Does the Road Look Wet on Hot Days?

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Reader Robert wrote in to say, “As I drive across the Panhandle of Texas, I am wondering why the highway looks wet and shiny off in the distance but normal as one gets closer.”

For the same reason that cartoon characters lost in the desert often think they see an oasis: a mirage caused by refraction. 

First, a quick physics lesson. Light moves slower through denser mediums and faster through less dense ones. As it travels through a given medium—say, air—it moves in a straight line. When it passes from one medium to another, though, and there’s a difference in density—say from air to water—the light waves change speed, which causes them to also change direction or refract, and then continue in a straight line on their new path.

An easy way to see refraction in action is to put a straw into a glass half-filled with water. From the top, it looks like the straw is bent or broken. From the side, depending on where in the glass the straw is, it might look like it grows wider below the water line or even detaches from the part above the water line. 

Remember that you see objects because light reflects off of them and then travels to your eye. What’s happening here is light from the straw is reflecting and traveling to your eye through two different mediums—the air and the water. Above the water line, light travels directly from the straw to your eye through the air and doesn’t refract (technically it travels through air into the glass and back into air, but the refraction into and out of the glass causes little enough deviation to not matter). Below the water line, though, the light reflecting off the straw has to travel through the water into glass and then into air. This light changes medium and speed, so it refracts or bends on its way to you. Your eye and brain don’t account for refraction when looking at the straw (stupid brain), and assume the object to be where the light waves appear to originate from along a straight line. The top and bottom parts of the straw are in line with each other, but the light from them comes along two different lines, making the straw look broken after your visual system gets done with it.

What Robert is describing is also the work of refraction. Maybe you were driving around one day and thought you saw a puddle on the pavement a little ways down the road. Once you got to the spot where you thought you saw the water, it was gone. Looking farther down the road, you see another puddle, but that one also disappears as you get closer to it. You can chase the puddles all day, but you’ll never actually find one. 

Light refracts not just when it moves through two different mediums like air and water, but also when it moves through different layers of the same medium that have different densities. As the sun beats down on the blacktop, it heats it up. The road, in turn, heats the air immediately surrounding it, keeping the air just above it warmer and less dense than the air farther up. 

As light from the sky travels downward toward the hot road, it moves through these increasingly warm and less dense layers of air, changing speed and refracting as it moves through each one. It winds up taking a sort of u-shaped path down toward the road, then parallel to it and finally back up into the sky—where it may meet the eye of someone standing up the road. 

When this refracted light reaches you, your brain and eye—like they did with the straw in the water—don’t account for all the bending it did along the way. They trace it back along a straight line and interpret that point as its origin and the location of the object. What you see, then, is a little bit of sky that appears to be sitting on the ground—an inferior mirage where the mirage is under the real object. Even as your brain and eye try to quickly make sense of what you’re seeing, the brain knows that sky on the ground doesn’t make sense, so you often wind up perceiving the mirage as water on the road reflecting the sky. Turbulence of the air also distorts the mirage, strengthening the effect. 

Sand, like highways, is really good at holding onto heat and warming up the air near it, so these types of watery mirages often happen in deserts and can fool people into thinking there’s water nearby.

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Big Questions
Do Bacteria Have Bacteria?
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Drew Smith:

Do bacteria have bacteria? Yes.

We know that bacteria range in size from 0.2 micrometers to nearly one millimeter. That’s more than a thousand-fold difference, easily enough to accommodate a small bacterium inside a larger one.

Nothing forbids bacteria from invading other bacteria, and in biology, that which is not forbidden is inevitable.

We have at least one example: Like many mealybugs, Planococcus citri has a bacterial endosymbiont, in this case the β-proteobacterium Tremblaya princeps. And this endosymbiont in turn has the γ-proteobacterium Moranella endobia living inside it. See for yourself:

Fluorescent In-Situ Hybridization confirming that intrabacterial symbionts reside inside Tremblaya cells in (A) M. hirsutus and (B) P. marginatus mealybugs. Tremblaya cells are in green, and γ-proteobacterial symbionts are in red. (Scale bar: 10 μm.)
Fluorescent In-Situ Hybridization confirming that intrabacterial symbionts reside inside Tremblaya cells in (A) M. hirsutus and (B) P. marginatus mealybugs. Tremblaya cells are in green, and γ-proteobacterial symbionts are in red. (Scale bar: 10 μm.)

I don’t know of examples of free-living bacteria hosting other bacteria within them, but that reflects either my ignorance or the likelihood that we haven’t looked hard enough for them. I’m sure they are out there.

Most (not all) scientists studying the origin of eukaryotic cells believe that they are descended from Archaea.

All scientists accept that the mitochondria which live inside eukaryotic cells are descendants of invasive alpha-proteobacteria. What’s not clear is whether archeal cells became eukaryotic in nature—that is, acquired internal membranes and transport systems—before or after acquiring mitochondria. The two scenarios can be sketched out like this:


The two hypotheses on the origin of eukaryotes:

(A) Archaezoan hypothesis.

(B) Symbiotic hypothesis.

The shapes within the eukaryotic cell denote the nucleus, the endomembrane system, and the cytoskeleton. The irregular gray shape denotes a putative wall-less archaeon that could have been the host of the alpha-proteobacterial endosymbiont, whereas the oblong red shape denotes a typical archaeon with a cell wall. A: archaea; B: bacteria; E: eukaryote; LUCA: last universal common ancestor of cellular life forms; LECA: last eukaryotic common ancestor; E-arch: putative archaezoan (primitive amitochondrial eukaryote); E-mit: primitive mitochondrial eukaryote; alpha:alpha-proteobacterium, ancestor of the mitochondrion.

The Archaezoan hypothesis has been given a bit of a boost by the discovery of Lokiarcheota. This complex Archaean has genes for phagocytosis, intracellular membrane formation and intracellular transport and signaling—hallmark activities of eukaryotic cells. The Lokiarcheotan genes are clearly related to eukaryotic genes, indicating a common origin.

Bacteria-within-bacteria is not only not a crazy idea, it probably accounts for the origin of Eucarya, and thus our own species.

We don’t know how common this arrangement is—we mostly study bacteria these days by sequencing their DNA. This is great for detecting uncultivatable species (which are 99 percent of them), but doesn’t tell us whether they are free-living or are some kind of symbiont. For that, someone would have to spend a lot of time prepping environmental samples for close examination by microscopic methods, a tedious project indeed. But one well worth doing, as it may shed more light on the history of life—which is often a history of conflict turned to cooperation. That’s a story which never gets old or stale.

This post originally appeared on Quora. Click here to view.

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Big Questions
Why Do Cats 'Blep'?
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As pet owners are well aware, cats are inscrutable creatures. They hiss at bare walls. They invite petting and then answer with scratching ingratitude. Their eyes are wandering globes of murky motivations.

Sometimes, you may catch your cat staring off into the abyss with his or her tongue lolling out of their mouth. This cartoonish expression, which is atypical of a cat’s normally regal air, has been identified as a “blep” by internet cat photo connoisseurs. An example:

Cunning as they are, cats probably don’t have the self-awareness to realize how charming this is. So why do cats really blep?

In a piece for Inverse, cat consultant Amy Shojai expressed the belief that a blep could be associated with the Flehmen response, which describes the act of a cat “smelling” their environment with their tongue. As a cat pants with his or her mouth open, pheromones are collected and passed along to the vomeronasal organ on the roof of their mouth. This typically happens when cats want to learn more about other cats or intriguing scents, like your dirty socks.

While the Flehmen response might precede a blep, it is not precisely a blep. That involves the cat’s mouth being closed while the tongue hangs out listlessly.

Ingrid Johnson, a certified cat behavior consultant through the International Association of Animal Behavior Consultants and the owner of Fundamentally Feline, tells Mental Floss that cat bleps may have several other plausible explanations. “It’s likely they don’t feel it or even realize they’re doing it,” she says. “One reason for that might be that they’re on medication that causes relaxation. Something for anxiety or stress or a muscle relaxer would do it.”

A photo of a cat sticking its tongue out
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If the cat isn’t sedated and unfurling their tongue because they’re high, then it’s possible that an anatomic cause is behind a blep: Johnson says she’s seen several cats display their tongues after having teeth extracted for health reasons. “Canine teeth help keep the tongue in place, so this would be a more common behavior for cats missing teeth, particularly on the bottom.”

A blep might even be breed-specific. Persians, which have been bred to have flat faces, might dangle their tongues because they lack the real estate to store it. “I see it a lot with Persians because there’s just no room to tuck it back in,” Johnson says. A cat may also simply have a Gene Simmons-sized tongue that gets caught on their incisors during a grooming session, leading to repeated bleps.

Whatever the origin, bleps are generally no cause for concern unless they’re doing it on a regular basis. That could be sign of an oral problem with their gums or teeth, prompting an evaluation by a veterinarian. Otherwise, a blep can either be admired—or retracted with a gentle prod of the tongue (provided your cat puts up with that kind of nonsense). “They might put up with touching their tongue, or they may bite or swipe at you,” Johnson says. “It depends on the temperament of the cat.” Considering the possible wrath involved, it may be best to let them blep in peace.

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