It's common wisdom that you should replace your toothbrush every few months or so. But the reason isn't so much that it gets ragged and crooked, but rather because toothpaste actually breaks down all the hook-like plastic on the bristles to a near-useless nub.
This video from Applied Science zooms up close on a toothbrush and dried toothpaste with an electron microscope to show how the process works. When dried out, you can actually see what toothpaste is made of: a mix of abrasives, gums, foaming agents, and detergents. But mostly, what strips plaque off your teeth are the abrasives.
Since those abrasives are made to not dissolve in water, it's pretty much like sand. And you're putting that on your toothbrush every day. Toothpaste is about equivalent to 600-grit sandpaper, which is enough to strip finish off of wood, and, if you're an industrious enough brusher, you could make it work for more than just teeth.
"If you're stuck somewhere and you don't have access to decent plastic polish, you can actually use toothpaste to clean up plastic headlight lenses on a car," Ben Krasnow of Applied Sciences said. That, and carpet stains. There's a life hack for you.