Defining an object's center of gravity, or center of mass, involves complicated physics terminology and ideas. You might notice that people often interchange the phrase "center of gravity" with "center of mass." For experiments in a constant gravitational field, such as earth's, the two phrases mean the same thing. In everyday language, an object's center of gravity describes the precise point where the forces acting on it balance out. This simply means that every object has a point of balance so it does not fall to one side or the other.
Lay a yardstick flat on a table. Make a fist with each of your hands and stick out the index and middle fingers of each, as if making a gun shape. Slide your outstretched fingers underneath each end of the yardstick and lift it off the table so only your four fingers support it. Slide your hands together until your index fingers touch; this will be the yardstick's center of gravity. Wrap a piece of clay around a random part of the yardstick and try the experiment again. No matter where you place the clay, your fingers will move to the center of gravity. If your fingers balance the same amount of mass as they slide, they will move at the same speed and meet in the stick's center of gravity. If one has less mass than the other does, it will slide more quickly and cover more ground until it balances out with the other at the center of gravity.
Position a chair, without wheels, with the back pressed up against a wall. Check that the chair cannot slide back any further. Sit a student in the chair and ask him to position the soles of his shoes flat on the floor and in front of the seat. Gently press your thumb or index finger on his forehead and tell him to stand. No matter how hard he tries, the student cannot stand because his center of gravity remains fixed over the chair rather than his feet; this throws his body off-balance so he cannot stand.
Turn a chair sideways and slide it against the wall. Choose one male student and have him bend over at a 90-degree angle. Place the crown of his head against the wall and position his feet in front of, but not under, the chair. Make sure he keeps his back as flat as possible and ask him to lift the chair, while remaining in the bent over position, and stand up. Repeat the procedure using a girl. The girl will lift the chair, but the boy will fall. This has to do with the position of a boy's center of gravity relative to a girl's. When the boy bends over the chair, his center of gravity is directly over it, so he needs the wall for balance. When he tries to lift the chair, he falls. When a girl bends over, her center of gravity is lower; when she stands, it remains over her feet. This keeps her in balance and allows her to lift the chair. Test this in a padded area in case a student falls during this experiment.