If you're on the orbiting International Space Station with a box of feathers and a box of sand, how could you tell which is which without opening them?

Poke or kick the boxes. The sand box is the one that resists the most

Explanation:

The one that resists the most a change in motion (inertia) is the one with the greater mass-the one filled with sand. Since we cannot open the boxes or perform any weight measurement we can distinguish them by their inertia. Sand has greater mass and thus greater inertia