system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 why can a person's weight change +/- 5-10 pounds a day when they definitely aren't eating 5-10 pounds of food/water a day. Where does this extra mass/wight come from?
gpt
Don't underestimate the weight of food and especially water. A half gallon of water (the standard-even-if-derided recommended daily amount) weighs over 4 pounds. A hearty meal can weigh 1-2 pounds on top of that. 

Not all of this material stays in the body long-term because we expel it as waste, but our body's capacity for storing water especially goes beyond the size of our bladders. Our cells literally get dehydrated overnight and will swell up with water over the course of the day.