A thousand years ago, if you threw something up, it came back down. Today if you throw something up, it’ll come back down. A thousand years from now, if you throw something up, it will come down. The law of gravity doesn’t change. (Toss something in the air to illustrate this.)
The truth of what is right and what is wrong is a law that doesn’t change either. What is right today was right a thousand years ago and will still be right a thousand years from now. Right and wrong don’t change.
Show the heart and explain that God wants you to follow Him with your whole heart. The wooden heart obeys gravity and slides down the rope from the top to the bottom. It goes all the way. Then talk about how people try to change right and wrong, how they try to justify sin and give away parts of their hearts to the things of the world. Suddenly the heart no longer obeys but stops in the middle.
“Some people don’t follow God with ALL their heart. They just go part way. They have ‘partial’ obedience. And partial obedience is complete disobedience.”
Of course, this is just a trick, but the truth is that God wants you to follow Jesus all the way, everyday.
There are many other uses for this illustration as well.