Another pinner says: How Many Drops of Water Can a Penny Hold? My daughter did this as her science fair project last year and it was very popular. She even used water variations (Kool-aid, salt water, etc).
Teach Junkie: 10 Scientific Method Tools to Make Teaching Science Easier - Foldable for the Scientific Method. A way to teach the Scientific Method in a way that that young learners will understand with hands on activities