I remember playing with this game back in high school. Was my first practical introduction to logical operators and binary. Ended up spending so much time trying to build some basic electronics once I found out how to build logic gates.

No idea if these still work, but kinda cool looking back on them, ten years later. Here's a fairly simple 4 bit adder with display and memory https://powdertoy.co.uk/Browse/View.html?ID=12853. Powder toy was never complete without some kind of explosive, so here's a grenade that proved quite popular https://powdertoy.co.uk/Browse/View.html?ID=6390.