I've read somewhere that you can get a very low-yield explosion by just dropping a big enough chunk of weapons-grade fissionable material onto another one. You only get several hundred tons of TNT equivalent of explosive force, but lots of radiation. It would make the perfect terror weapon.

This is essentially how louis slotin died: http://en.wikipedia.org/wiki/Louis_Slotin

You either read or remembered wrong, or that source was referring to something like an explosives-grade uranium dart being dropped through one or more explosives-grade uranium donuts and from a great height. That method would not work for plutonium, which -- because of its greater propensity to prematurely generate neutrons -- requires the more-rapid mass-assembly method of an implosion.

Ah, I remembered, but not specifically enough. It was Uranium, but I forgot that it had to be darts through a donut, and I didn't know it had to be a great height.

Is that 2 stories? 10 stories?