Two cats on a roof: Which one falls of first?
The one with the smallest mew!
Let's face it, Physics jokes are just not funny: They're more likely to make you groan (if that) than laugh. But does that mean we shouldn't tell them? Well, if our job was to be a comedian, then no. But it isn't - our job is to teach Physics: to make it memorable. And whilst Physics jokes may be bad, the fact that they are so bad (and corny) often makes them very memorable.
When I first created my website, QuantumBoffin, someone commented to me "QuantumBoffin: That's a stupid name!"
I replied "Well you remembered it didn't you?"
By the way, why was six scared of seven?
Because seven ate nine.