That really would make me laugh. I love when children show advanced reasoning like that.
In my following story, his reasoning was wrong, but it was funny! I was babysitting a 5 year old who showed me how high he could jump. I told him I didn't think I could jump that high and showed him by barely propelling myself off the ground. He looked up at me and said, "I know why you can't jump as high as me, you're too heavy!"