Are you compressing them together, prior to the load? Also, it is sometimes a good idea to guide their hand...helping them to clench around the sponges. Assist them in turning their hand palm down, afterwards. This will make it a little more difficult for them to prematurely open their hand and discover the extra sponge. Once their hand is in the palm down position...you'll be able to notice if they start to turn their palm upwards. Quickly remind them to keep their hand palm down. It's really all about audience management. Learn to control the situation.