Q: Why can’t a nose be 12 inches long? 
A: If it was, then it would be a foot!

Explanation: In the standard system of measurement, the system that is used in the United States, 12 inches equals one foot (about 30 cm).  A foot is also a body part, just as a nose is a body part.  This joke plays on the double meaning of the word “foot.”  And of course, a nose cannot be a foot!