Q: Why does Santa go down the chimney on Christmas Eve?

Q: Why does Santa go down the chimney on Christmas Eve?
A: Because it 'soots' him!

This is another pun. To suit someone means to fit them, or to be appropriate to them. Though I have never heard soot used as a verb, if soot were to be used as a verb, it would likely mean to cover in soot.

As far as puns go, this is a pretty good one.