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How Do Smoke Alarms Work?

The smoke alarms in my apartment building are both ridiculously sensitive and ridiculously loud. They regularly go off even when there’s no smoke, and I often have to scramble up on top of a chair to reset them because a pot of boiling water is producing steam. If I stand in the hallway for a few minutes just before dinner time, I can hear them go off in the different apartments, followed by my neighbors’ strings of profanity and rushed footsteps. This got me wondering: How do these things work?

Where There's Smoke ...

There are two main types of smoke alarms used in homes: ionization detectors and photoelectric detectors. Inside the ionization ones there’s an ionization chamber with two plates and a source of ionizing radiation. The alarm’s battery sends a voltage to the plates, charging one positive and the other negative. The radiation source, a small amount (around 1/5000th of a gram) of an isotope called Americium-241, decays and emits alpha particles (subatomic particles made of two protons and two neutrons) at a reliable, constant rate. As the particles travel through the chamber, they ionize, or knock an electron from, the oxygen and nitrogen atoms in the air that passes through the chamber.

The newly free electrons, which have a negative charge, are attracted to the positively charged plate, and the now-positively charged atoms are attracted to the negative plate. This maintains a small but constant current between the two plates. When smoke enters the chamber, it disrupts this little dance of ionization and lowers or kills the current between the plates, triggering the alarm.

Now, if the idea of radioactive isotopes hanging from the ceilings in your home alarms you a little, and quick death by fire sounds more attractive than slow death by radiation, let me put your mind at ease. Alpha particles have very little penetrative power. They can’t get through the plastic of the detector, and if they did escape, they can’t travel very far in regular air. Because of the small amount of Americium in there, and the design of the detector, there’s no health hazard unless you monkey with the chamber and directly expose yourself to the particles (that is, inhale or ingest them).

The two big drawbacks to ionization detectors are that the radioactive isotope requires proper disposal of old detectors so that they don’t pose a hazard, and that their design is very sensitive (to detect hot, fast fires that produce very little smoke). This means, as I can attest, that they’re prone to false alarms caused by dust and steam and other vapors.

Ray of Light

The other common kind of detector, a photoelectric detector, contains a light-emitting diode that sends a beam of light across the top of a T-shaped chamber. At the base of the T is a photocell that detects light. When smoke enters the chamber, the light hits it, gets scattered into the base of the T and strikes the photocell.

When a certain amount of light hits the cell, it triggers an electrical current that sets the alarm off. These detectors are not as sensitive as the ionization ones and are designed to detect slow, smoldering, smokier fires.

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Big Questions
What's the Difference Between Stuffing and Dressing?
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For carbohydrate consumers, nothing completes a Thanksgiving meal like stuffing—shovelfuls of bread, celery, mushrooms, and other ingredients that complement all of that turkey protein.

Some people don’t say “stuffing,” though. They say “dressing.” In these calamitous times, knowing how to properly refer to the giant glob of insulin-spiking bread seems necessary. So what's the difference?

Let’s dismiss one theory off the bat: Dressing and stuffing do not correlate with how the side dish is prepared. A turkey can be stuffed with dressing, and stuffing can be served in a casserole dish. Whether it’s ever seen the inside of a bird is irrelevant, and anyone who tells you otherwise is wrong and should be met with suspicion, if not outright derision.

The terms are actually separated due to regional dialects. “Dressing” seems to be the favored descriptor for southern states like Mississippi, Tennessee, South Carolina, and Georgia, while “stuffing” is preferred by Maine, New York, and other northern areas. (Some parts of Pennsylvania call it "filling," which is a bit too on the nose, but to each their own.)

If “stuffing” stemmed from the common practice of filling a turkey with carbs, why the division? According to The Huffington Post, it may have been because Southerners considered the word “stuffing” impolite, so never embraced it.

While you should experience no material difference in asking for stuffing or dressing, when visiting relatives it might be helpful to keep to their regionally-preferred word to avoid confusion. Enjoy stuffing yourselves.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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Big Questions
Why Do the Lions and Cowboys Always Play on Thanksgiving?
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Rey Del Rio/Getty Images

Because it's tradition! But how did this tradition begin?

Every year since 1934, the Detroit Lions have taken the field for a Thanksgiving game, no matter how bad their record has been. It all goes back to when the Lions were still a fairly young franchise. The team started in 1929 in Portsmouth, Ohio, as the Spartans. Portsmouth, while surely a lovely town, wasn't quite big enough to support a pro team in the young NFL. Detroit radio station owner George A. Richards bought the Spartans and moved the team to Detroit in 1934.

Although Richards's new squad was a solid team, they were playing second fiddle in Detroit to the Hank Greenberg-led Tigers, who had gone 101-53 to win the 1934 American League Pennant. In the early weeks of the 1934 season, the biggest crowd the Lions could draw for a game was a relatively paltry 15,000. Desperate for a marketing trick to get Detroit excited about its fledgling football franchise, Richards hit on the idea of playing a game on Thanksgiving. Since Richards's WJR was one of the bigger radio stations in the country, he had considerable clout with his network and convinced NBC to broadcast a Thanksgiving game on 94 stations nationwide.

The move worked brilliantly. The undefeated Chicago Bears rolled into town as defending NFL champions, and since the Lions had only one loss, the winner of the first Thanksgiving game would take the NFL's Western Division. The Lions not only sold out their 26,000-seat stadium, they also had to turn fans away at the gate. Even though the juggernaut Bears won that game, the tradition took hold, and the Lions have been playing on Thanksgiving ever since.

This year, the Lions host the Minnesota Vikings.

HOW 'BOUT THEM COWBOYS?


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The Cowboys, too, jumped on the opportunity to play on Thanksgiving as an extra little bump for their popularity. When the chance to take the field on Thanksgiving arose in 1966, it might not have been a huge benefit for the Cowboys. Sure, the Lions had filled their stadium for their Thanksgiving games, but that was no assurance that Texans would warm to holiday football so quickly.

Cowboys general manager Tex Schramm, though, was something of a marketing genius; among his other achievements was the creation of the Dallas Cowboys Cheerleaders.

Schramm saw the Thanksgiving Day game as a great way to get the team some national publicity even as it struggled under young head coach Tom Landry. Schramm signed the Cowboys up for the game even though the NFL was worried that the fans might just not show up—the league guaranteed the team a certain gate revenue in case nobody bought tickets. But the fans showed up in droves, and the team broke its attendance record as 80,259 crammed into the Cotton Bowl. The Cowboys beat the Cleveland Browns 26-14 that day, and a second Thanksgiving pigskin tradition caught hold. Since 1966, the Cowboys have missed having Thanksgiving games only twice.

Dallas will take on the Los Angeles Chargers on Thursday.

WHAT'S WITH THE NIGHT GAME?


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In 2006, because 6-plus hours of holiday football was not sufficient, the NFL added a third game to the Thanksgiving lineup. This game is not assigned to a specific franchise—this year, the Washington Redskins will welcome the New York Giants.

Re-running this 2008 article a few days before the games is our Thanksgiving tradition.

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