This object lesson teaches that you need the Holy Spirit to minister effectively to others. If you don't spend time with God each day to get filled with His Spirit, you will eventually run dry and have nothing left to offer anyone else.
The lesson involves repeatedly filling a Styrofoam cup with water and then piercing the cup to let the water drain out. The cup represents us. The water represents Living Water, i.e., the Holy Spirit. And the holes in the cup represent sin in our lives that allows the Spirit to leak out.