Ever thought of something that seemed fairly sci-fi? It doesn't have to work in real life, hence the "fi" part

I was sitting upstairs and watching Futureweapons, and I started thinking about the way Minesweepers search for mines, they look for metal, and things that don't belong, and I thought about a chemical reaction I'd seen on TV once:
Rubidium when dropped in water creates a fairly large explosion, so...
Rubidium mine: A glass container with water in the bottom, and a sealed separate compartment with a fairly small piece of rubidium (less than 2 cubic inches would probably do)
when enough weight is put on it an explosion about the size of a hand grenade would be created, of course, I don't know how long the reaction requires - but it seemed clever at the time

Ever think up something that seemed pretty sci-fi? It doesn't have to be military like the above, just something that could possibly be made in the future, or at least look like it