Q: What fruit do you eat when you are sad?
A: Blueberries.

Explanation: In English some colors are associated with feelings.  Blue is associated with sadness.  When you “feel blue” it means that you are feeling sad.

If the berries are blue then are they sad?  Only in this joke are berries sad; it is the double meaning of the word ‘blue’ that makes this joke funny (the color and being sad).

Blueberries for me are not sad- they make me happy, especially when I pick my own in the wild.  Here is a video of some campers picking blueberries in the Boundary Waters: