Your cousin comes by to hang during summer, and both your mom's are hanging by the pool - your cousin has grown up a lot since you last saw her! And she's really coming out of that tank top. WHOA! And her bush is really coming out from those shorts! She sees you looking feels insecure because her mom is always telling her to keep it contained. But you're comfortable with her and don't mind, telling her she looks great. This gives her so much more confidence then she's used to, and she decides maybe you two should just... hang out upstairs. In bed. Naked.