In the fairy tale, Rapunzel has hair so long it reaches all the way to the ground from high in the tower she is trapped in.
Now I've never seen anyone's hair grow quite that long, but why is it that the hair on our heads can grow to amazing lengths, while hair on other parts of the body never gets very long?
Let's compare the hair on your head with the hair on your arm. These two types of hair have a lot in common. Both grow actively for a certain period of time, then hang out in the follicle for a while after they've stopped growing. Eventually, they'll fall out and be replaced by new hair.
Despite these similarities, there's one important difference: the length of the growing period.
Arm hairs grow for a few weeks or months at the most, but the hairs on your head have a much longer growing period, and can keep getting longer for years. Hair seems to grow continuously at a more or less constant rate, because individual hairs begin the growing period at different times.
In fairy tales, Rapunzel definitely wins the prize for the longest hair, but when it comes to real people, the competition's skewed from the start. That's because your genes play a role in how fast your hair grows, and thus how long each strand can possibly get before its growing period ends.