Lego General Bombs - this series of bombs (1 being the smallest, 3 the largest) represents a cheap way to add some stored energy to a simple Lego structure. When the trigger (the pillar in the middle of the bombs) strikes the ground, the backbone of the bomb is weakened followed by violent fracturing. AKA, explosion.
The picture below is a flash photograph taken at the moment of impact of LGB2. You can see the backbone of the bomb beginning to fracture!
Ongoing work is focused on delivery platforms for these explosives. Read more details at http://legoheavyindustries.blogspot.com/2014/11/lego-general-bombs-1-2-and-3.html.