Ask the children to press down on the bags, lean on them, and rest objects on top of them. Pupils may mention inflated beach toys, air mattresses, and tires. Have the children untie the bags, put them down inside the jars with the mouth of each bag folded over the mouth of the jar and again tie them on tightly. Air under pressure is used to operate many devices that require great force. Car lifts in service stations use compressed air.
Air brakes on a truck apply far more force that drivers can with just muscles.
Air-filled tires support cars and trucks. Because air is a fluid, it can enter small openings and fill spaces of all shapes easily. Also, the pressure at any point in confined air is the same. Thus, air can be squeezed into a tire through the tiny opening in the valve stem. Once in the tire, air presses on all parts of the tire equally.