For me, it's when my girl says, "I need a tummy rub," all cute and innocently. And then when I lower her pants to let her belly out, say goes, "No, I'm gonna look huge." To which I reply, "But babe, you are huge. Now do you want that belly rub or not?" She always relents and lets me massage away her food babies.
Speaking of which, another one she always gets me with is, "I hope you plan on marrying me, cause I'm pregnant with your food baby." Sure sign of a good time.