Rich asked me this question the other day.
I said 'what do you mean, what else would they be?' I couldn't believe he was serious. Then I realised he was being genuine so I added a rusty piece of science to help him along, wisely explaining that the difference between fruit and vegies can be established by whether they had seeds or not i.e. no seeds = vegies, seeds = fruit. To this he exclaimed 'Does that mean watermelons are fruit?'.
This had me in fits of laughter as Rich has always had a funny frui/vegi dyslexia. He gets very confused by things at the supermarket esp when I request he find me something like a cucumber or a mandarin. Turns out he thought that watermelons were simply 'melons' not fruit. I'm not sure what he thought grapes would be if they wern't fruit.
The seeds/no seeds explaination did not help things. He proceeded to get upset about pumpkins and tomatoes being fruit (understandable I guess).