Kneel down, then rest your bottom on your heels. Crouch down, arms together extending forward, palms touching each other, and put your elbows to your knees. Have a friend place a foam cup where your fingers reach. Now, put your arms behind your back, stay on your knees, but reach toward the cup with your nose and try to knock it over. If you are going to fall, CATCH YOURSELF WITH YOUR HANDS. Many (not all) girls can knock over the cup with ease, while many (not all) boys cannot.
If your body's center of gravity (or, COG) extends too far over your knees when you lean forward, you will feel yourself tip over. If your body's COG stays behind your knees, even when leaning forward, you won't tip. Many (not all) girls have a COG lower in their body, toward their hips. Many (not all) boys have a COG higher in their body, toward their shoulders.