How is it possible to gain 15 pounds in one day?

Be really dehydrated and have an empty stomach prior and totally pig out. Most people's weight already fluctuates by around 5 lbs throughout the day.

Either the hugest binge I have ever seen, water weight or the person weighed themselves at a different time of day as they usually do (or on a different day entirely) as our weights aren't consistent. However 15 is a lot :O Maybe the scales are just dodgy.

Be that guy who ate 70+ hotdogs. How much would the weigh anyway?

Retaining a lot of fluid is all I can think of.

Murder a toddler and wear them as a hat.

Eat some concrete. That should do it. :-)

Carry change in your pockets!

water and food weight.

Move to Jupiter

Water and food.