This one isn't hard at all. I unwittingly gained a bunch of weight in one period because I was busy at work and too lazy to make proper food, so I just ate a lot of peanuts as snack food, 'cause I'd heard they're somewhat healthier than other snack foods. Peanuts are basically nothing but protein+fat, but I gained a bunch of weight because they're so calorie-dense (one 8-oz bag has like 1500 calories). I'm not 100% sure, but I think the fat+salt aspect might promote overeating; I can't imagine eating enough walnuts, say, to gain weight, but I can munch on salted/roasted peanuts all day.