Have a volunteer come to the front of the class. Give them the hard piece of play dough and tell them to shape it into what they want. When they aren't able to, ask them why (it's too hard, can't mold it the way they want...). Give them the soft play dough. Tell them to mold or shape this one the way they want to. When they are done 'playing', ask the class to compare this to humility. Explain to them that if we are stubborn and 'hardened' like the play dough, and do not rely on the Lord, He will not be able to use us as He intends to.
If we are humble and teachable, the Lord will be able to mold us and shape us into what He wants us to be, and we will fulfill our potential.