Snuffy in the "Aha she cried and waved her wooden leg" thread posed this question

"What is the difference between a duck?"
A. "One of its legs are both the same"

I recall this one from teachers college in the 70's; along with:

Q. Why are Volkswagens?
A. Because motorbikes have no doors

Q. Why are apples round?
A. Because bananas are yellow.

and

Q. Why do mice spin in pairs?
A. The higher the fewer.

Can anyone recall any others?