How is it the scale says I gained 2 pounds when I only had 800 calories that day?

Scales can be wrong, and you sometimes weigh more/less in the mornings/evenings.

Make sure that when you weigh yourself you are either naked, or in really light clothing. Also, make sure that you are weighing yourself every morning, after the use of the restroom, to eliminate water weight and other things that add weight.

Weighing scale does not measure the amount of calories you ate. in its most basic sense, the weighing scale measures the pull of gravity on you (weight - pull of gravity in an object , mass - amount of matter in an object)

So, it pretty much measures everything. Simpleton is right, you might want to weigh just yourself, no water, no stool, no clothes whatsoever. =)

Also, it’s nice to know that you will weigh less when you’re on top of the mountain than when you’re down in the cities. =)

I’d firstly like to start by saying that 800 calories a day is not anywhere near enough for you to be consuming. When you do this your body goes into starvation mode and actually starts storing fats, which will give you the opposite effect of what you’re aiming for. If you’d like to be losing weight, you need to do so in a healthy manner and consume at least 1200-1500 calories a day. Exercising in a combination with a healthy diet is the key.

As for your question, usually it your weight will fluctuate throughout the day. Due to this it is recommended that you weigh yourself every day at the same time, usually in the morning right after you wake up. This will give you the best answer.