When you exercise, you burn as fuel.
endorphins
calories
oxygen
water (H20)

Calories is the answer;

Endorphins are the chemicals released when you are exercising, they normally make you happier.

Oxygen is what we use to breath, we don't burn it for fuel.

Water is what we drink, and is what our bodies use to produce sweat while exercising.

This leaves with your answer, which is calories.

I hoped this helped you out.

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