Hair is pretty amazing stuff. It sprouts from almost every inch of skin on our bodies—only the palms of your hands, the soles of your feet, and your lips are officially fuzz-free. Apparently, the average person grows 590 miles of hair during their lifetime. But have you ever wondered why your eyelashes and eyebrows don’t just keep on growing until they’re as long as the hair on your head?

Most of us resign ourselves to spending money on regular haircuts, but happily our brows and lashes are much lower maintenance. They politely grow to a decent length and stay there. Of course, there are exceptions. You can probably think of at least one person you know of who is referred to as "that guy with the Dennis Healey eyebrows." But for the vast majority of us, our eyebrows and eyelashes will never grow to a length that requires a special trip to the hairdresser.

How Hair Knows When to Stop Growing

There are three phases to a hair follicle’s growth cycle: the anagen (growth) phase, the catagen (transition) phase, and the telogen (resting) phase. Around 85 percent of the hair on your head is in the anagen phase at any one time. During this time, the follicle grows new cells and the hair strand gets longer at a rate of around 15 centimeters per year.

When it’s in the catagen phase, the hair follicle shrinks to around a sixth of its original length, causing the strand of hair to detach from the blood supply and push up towards the skin’s surface. Around two weeks later, the telogen phase takes over and the hair lies dormant for several months before the cycle starts again. At this stage the strand of hair can be very easily pulled out. Around 10-15 percent of the hairs on your head are in the telogen phase at any given moment, so there’s no need to panic if you lose a fair few strands during brushing or washing. (Though if it starts coming out in handfuls it might be worth a visit to a doctor).

Depending where on your body the hair is located, the anagen phase can vary greatly, from around 30 days for an eyelash to up to an average of six years for a hair on your head. The longer a hair stays in the anagen phase, the longer it can grow. As lashes and brows have a much shorter anagen phase than head hair, they don’t get a chance to grow as long before they are shed. Scalp follicles are different from those on the other parts of your body, as they use recycled proteins or excess proteins your body doesn’t need to produce hair for an extended length of time.

The exact length of each phase in the hair’s growth cycle is slightly different for everyone and varies according to your genetics. Some particularly hirsute individuals have follicles with a longer anagen phase than the norm, causing them to grow unusually long hairs on various parts of their body.

The Guinness world record for the longest eyebrow hair is held by a Japanese man named Sumito Matsumura and was measured at 18.1 centimeters, while the world’s most lengthy eyelash is credited to Stuart Muller of Florida. Sprouting from his left upper eyelid, Stuart’s record-breaking eyelash measured an impressive 6.99 centimeters.

However, both these records pale into insignificance alongside the world’s longest head hair. According to Guinness, a Chinese woman named Xie Qiuping stopped cutting her hair in 1973 and by 2004 it measured an astonishing 5.627 meters. This means that if she stood at a second floor window and let down her hair Rapunzel-style it would touch the ground. While the records don’t state her reasons for letting her hair get this long, we hope she wasn’t holding out for a handsome prince for the entire 31 years.