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The power rocket that was used in Lesson 9 of Spring 2006 KidsOwn Worship works best when it is filled with water at a level of only 1/3 to 1/2 full. Any more water than that, and you're likely to get your hands wet. Also, the instructions indicate not to pump the air pressure pump more than 20 times, but I would suggest 10 times at most.
If that doesn't work, also try lubricating the air pressure pump with light oil. A little WD-30 works well; a basic sewing machine oil would work okay too.
Hope that helps! Thanks for all you do to minister to kids for God's kingdom. :-)