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Developed for the NISE Network with funding from the National Science Foundation under Award Numbers 0532536 and 0940143. Any opinions, findings, and conclusions or recommendations expressed in this product are those of the authors and do not necessarily reflect the views of the Foundation.
We've had a lot of fun letting visitors put their hands into apparently empty beakers of water only to find they're half full of lumps of superabsorbent polymer "crystals".
Some of our volunteers call them "ghost brains", because you can't see them, and they're squishy and gross.
One variant of this activity is to put both liquids in the same container instead of separate containers. The oil will float on top of the water, so you can have objects half in water and half in oil, half visible and half invisible.
When doing this, transferring objects into oil from water can spoil the illusion. Water sticking to the surface of the object can make it visible.
You can extend the density column idea with activities like Density Rainbow: