I'm not sure. But I think the two things just go hand-in-hand. When I met my first tickle partner, I thought that I just wanted to tickle her feet more than anything else. But she laughed so much when I tickled her sides just above her hips, that it became my favorite place to tickle. So I realized that it was her laughing that I really liked. But you can't have one without the other, so who knows...