How do balloons get filled with air?

Balloons are filled with air by inserting a nozzle or the opening of the balloon into a source of air, such as a person's mouth, a pump, or an air compressor. When the air is blown or pumped into the balloon, it expands, causing the rubber or latex material to stretch. Once the desired size is reached, the opening is sealed, usually by tying a knot, to keep the air inside.