Why is a cook’s brain-pan like an overwound clock?
They’re only good for short order?
Smacking it doesn’t help, but it sure is fun.
Heh heh.
I suspect a good answer might involve the word “thyme.”
Because they’ve both got moustaches, except the clock? 
[Inspired by the joke:
What have Kenny Dalglish and Graeme Sounness got in common?
They’ve both got a moustache, except Kenny Dalglish]