Magic Sand.
It's really stupid.
We got a box for dd1s birthday - inside were several packets of coloured sand. On the box are amazing underwater sculptures.
The reality - piles of damp sand and water everywhere, a small flood under the table, damp sleeves, piles of glowing sand. A bowl full of luminous heaps. There were no tools included - no clue as to how to make the amazing structures.
There is 5 minutes of amazement at how the sand is kind of pliable in the water, then dry when you get it out, but when that dies down, its a pain in the arse.
Anyone else got any tips on things not to bother with?