The faster you move, the heavier you get. You're heavier when you run than when you're standing still.

The more energy you put into something, the more mass it accumulates. So if you're moving fast, you gain weight. It's only a tiny, temporary weight gain, but if you were traveling close to the speed of light your mass would increase rapidly.

Other weird physics facts:
http://www.telegraph.co.uk/science/6546462/The-10-weirdest-physics-facts-from-relativity-to-quantum-physics.html