Minecart Blood
In Just Blood Carts, when a minecart hits a mob at high speed (including peaceful mobs) it turns the mob into blood, which collects in the minecart.
When a minecart is full of blood, and goes down a hill, at high speed, the blood will fly out of the minecart and turn into a block of Congealed Blood.
You can detect whether a minecart is full of blood by running it over a stone pressure plate.