Spiders don't have an internal skeleton like we do. Instead, their muscles are anchored to an exoskeleton—a sort of hard, semi-flexible shell that encases a spider's whole body. In order to grow bigger, spiders have to grow new exoskeletons and shed old ones.
Karli Larson found a spider on her window frame in the process of shedding its exoskeleton. Naturally, she filmed it and set the whole thing to music. She says:
The entire molting process took about 30 minutes to fully complete. This is the interesting part, sped up.
The camera is a little shakey, so if that bothers you, well, sorry. But I think this is still way fascinating.
Read more about spiders, their exoskeletons, and the molting process at HowStuffWorks
Thanks, Maggie Ryan Sandford!