This video is about a year old, but it's resurfaced thanks to David Grann's always-excellent Twitter feed:

"A single drop of venom (from a Russell's viper) is dripped onto a petri dish of blood, and in seconds the blood clots into a thick chunk of solid matter." That about sums it up. Plus you get to watch venom drip out of a viper's fangs.

And it's not all morbid, either: Here's a short segment from PBS looking at whether the clotting agent in snake venom could be harnessed to stop people from bleeding to death after an accident.