Introduce the concept of gravity to students by explaining that it is a force that acts on objects. Objects will fall to the ground at the same rate -- 9.81 meters per second or 22 miles per hour -- unless they are acted upon by another force. A book dropped from the ceiling, for example, will stop falling if it is acted on by the force of a desk that catches it. Help students understand that even though gravity is invisible, it is a force similar to pushing, throwing or shoving.
A central goal of teaching students about gravity is to help them understand that gravity acts on every object with the same force regardless of how big that object is. To do this, use a large object such as a heavy book and a small object such as a marble. Ask students which one they think will hit the ground first. Most will say the book because it is heavier. Then, stand on a chair and drop the objects at the same time. Students will see that the objects hit the ground at the same time. Explain to them that even though they can't see it, gravity is pulling on each object with the same force.
The next goal of a gravity lesson should be to teach students about how other forces can act on an object simultaneous to gravity. Do this by dropping a piece of paper while standing on a chair. The students will notice that the paper drops much more slowly. Resistance from the air causes the paper to fall slowly. Conclude this experiment by bunching the paper up into a ball and showing students that it falls much more quickly this way, even though its weight hasn't changed. This is because the ball meets less air resistance.
Some students will wonder why everything in the world doesn't keep falling all the time. This is an excellent opportunity to teach students about inertia. Inertia is the tendency of an object to remain at rest if it is already at rest and to remain moving if it is already moving. Explain that once gravity is counteracted by the force of something catching an object, it stops moving and inertia keeps it still.