Unfortunately, I must admit that I almost understand the joke =-S only because I've heard the (dare I say) "correct" version: Q: What's the difference between a duck? A: One of its feet are both the same. The humour being precisely the fact that it does not make sense ... the listener is supposed to be confused after hearing the question.

whats the difference between a duck. and what excatly??! joke would have been funnier if it made sense...

i kno its stupid my cousin entered it!

if you ask "whats the difference between..." there has to be 2 things. It cant be just "a duck" because youre comparing them. Learn to be funny and actually make sense, and not submit homemade jokes