Balance

Credit: wiki.brown.edu

Center of Gravity if an interesting concept that children can explore at home and classroom.
Check the video:


More:
Balance & Center of Gravity
http://www.algarcia.org/AnimationPhysics/BalanceTutorial.pdf
http://web.mit.edu/jabbott/www/physicsofbalance.html
Equilibrium & Torque
http://www.physicsclassroom.com/class/vectors/u3l3c
http://hyperphysics.phy-astr.gsu.edu/hbase/torq.html