Can you gain five pouds in one day?

Yes, but it's not exactly accurate to weigh yourself daily. It all depends on what time of day you weigh yourself (weirdly enough), if you've just eaten a meal (you're guaranteed to be a few pounds heavier after you've eaten dinner, trust me), or what clothes you are wearing. If you gain 5 lbs. in a day, you're probably not gaining real weight, it might just depend on if you're retaining water, or if you just finished exercising. Mostly, if you gain weight, you'll gain over slow, long periods of time. If you want a really accurate daily weighing session, try this:

-take off your clothes when you weigh yourself

-do it at the same time of day each day

-don't do it right after you eat or right after you exercise

for example, what i do is i weigh myself around once a week to see if I've gained or lost. in the morning i stand on the scale in my undergarments right before I've showered and eaten breakfast. right when you get up, you have no food in your body, so that's usually closest to your real weight, anyway.