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
The liquid is called condensation and its generally formed when water vaor in the air cools down rapidly and collects around or on an object forming water droplets.