Just one if it's Professor X.
An x-man.
Because dogs can't whistle. (X-post from r/dadjokes)
You don't know man, you weren't there!
Just one, if you put it in the right place.
Allah them. (I googled several varations and thus far I believe I am the originator)
None, because they keep on asking why all of the other light bulbs in the house aren't being changed at the same time.
Professor + I'm sorry..
Professor +