reposting here since /sci/ is slow

The scientific way to lose weight
Forget about pills, diets and exercise. We're going to use fucking thermodynamics.
Your fatass body is a heat engine, designed to keep mammals warm and alive.
Every day you lose heat to maintain that warmth, this burns calories but not enough to lose weight.
Now we have a way to take that little engine to overdrive.
Fill a bathtub with cold water(cold but not too cold), get in for 6 hours and enjoy.
In the time you'll lose calories like extreme full day exercise but without moving a muscle.

I am underweight, do you have any advice on how to easily gain weight?

I'm not going to set myself on fire, since excess heat actually does not generate calories in your body.