This isn't a joke I heard but rather one I pulled on someone a few years ago.
I just came back from a shopping trip for my birthday present. I walked into the kitchen carrying it. I noticed my sibling in the kitchen and said "Hey 'Fred', you wanted a tank for your birthday right?" as I held up a 10 gallon fish-tank kit. One of 'Fred's' (not his real name) interests were the military tanks, rather than my pretty glass aquarium.