Volcano katas! Shuffle the caps until all the green X's are covered, and none of the red X's are. Caps can only move such that they don't cause an explosion. A few puzzles to start with:
You can get the caps into the corner easily: A piece in the middle is worth a lot: Order matters:Regrettably, due to the Java applet security policy, it's not possible to copy and paste with the text field. I am working on some workaround for this, at the very least, a downloadable version of the program.