Density is omnipresent in the processes of everyday life, explaining everything from ice floating in a drink to balloons drifting through the air. Density is a relative system of measurement which shows whether one substance or object will rise or sink in a given medium.
Everyday instances in which density is important include the flight of lighter-than-air vehicles, flotation, swimming and boating, and cooking and baking.
Zeppelins fly by using gases such as hydrogen and helium to render themselves lighter than the air around them. This causes them to ascend through that medium until they reach a thinner strata of atmosphere and find equilibrium or until they descend by adding ballast or venting lift.
During food preparation, heat causes moisture to sublimate into steam which is less dense than the surrounding atmosphere. Steam also carries heat away from food, helping it cool off once a meal is done. It plays an important part in regulating heat within prepared food and in processes like frying and fricasseeing.
Water's density is such that most humans can float in it, their own density being lesser. This, and the resistance afforded to strokes by water's density, allows for swimming. Boats, equipped with internal flotation devices and open spaces filled with air, float partially on the surface of water.