Filtration is the process of separating solid particles from a liquid or gas by passing the mixture through a material with tiny holes. When you make coffee, hot water flows through ground coffee beans and a paper filter, which catches the grounds while letting the flavored liquid drip into your cup. That's filtration in action.

Filtration happens all around you. Your home's air conditioner uses filters to trap dust and pollen before circulating clean air through your rooms. Car engines have oil filters that catch dirt particles to keep the engine running smoothly. Water treatment plants use massive filtration systems with sand, gravel, and special membranes to remove impurities, making water safe to drink.

The key to filtration is the filter itself, which acts like a specialized strainer. The filter's holes must be small enough to catch unwanted particles but large enough to let the liquid or gas flow through. A coffee filter has holes perfect for blocking coffee grounds but letting water pass. A surgical mask filters out larger particles from the air you breathe. Your kidneys perform biological filtration, removing waste from your blood while keeping the good stuff your body needs.

Scientists and engineers design filtration systems for countless purposes: purifying pool water, cleaning aquarium tanks, manufacturing medicines, and processing chemicals in laboratories.