Aglet

That little plastic thing at the end of the shoelace. It also grows back the hair of a bald person. Chances are if you watched that one Phineas and Ferb episode, you know what this is from memory.

Say, what did Phineas and Ferb say that thing at the end of a shoelace was called?

It's called an aglet. A-G-L-E-T! Don't forget it!