Q: Why was Pavlov's hair so soft?

Q: Why was Pavlov's hair so soft?
A: He conditioned it.

This is a pun. The first thing that most people know about Pavlov is that he conditioned dogs to salivate when they heard a bell ring.

To condition hair normally means to apply conditioner to it, but in the case of Pavlov, he might have trained it to be soft when he rung a bell or something like that.