You throw me out when you want to use me and you take me in when you don't want to use me. Who am I?

You need a key to receive an answer from me. The answer you'll find is straight from your mind. What is it?

What contains zero calories, but if you consume it, you will probably gain weight?

What do you purposefully put lots of in and on your body, but run away from when you encounter it outside?

What do you have when you're sitting down that you don't have when you're standing up?