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:
All the matter that makes up the human race can fit into a sugar cube. This is because atoms are mostly empty space. If you crammed all the atoms in all human beings together, without any wasted space, they wouldn't be any bigger than a sugar cube. However, that sugar cube would weigh 5 BILLION TONS.
Light doesn't always travel at the speed of light. The speed of light is only constant in a vacuum. When it passes through matter it can slow down. Photons (light particles) pass through water at 3/4ths the regular speed of light. Light was once slowed down to 38 miles per hour.