Damon's buddies Ben Affleck and Jimmy Kimmel nominated him last week -- and although it took him a little while to take the challenge, he had a good reason: He didn't want to waste clean water by dumping it on his head.
So instead, the 43-year-old actor decided to use toilet water.
"This is truly toilet water. I've been collecting it from various toilets around the house," said Damon, who co-founded Water.org, which helps provide safe water and sanitation in developing countries. "For those of you like my wife who think this is really disgusting, keep in mind that the water in our toilets in the West is actually cleaner than the water that most people in the developing world have access to."
Damon donated to the ALS Assocation and also nominated three people for the challenge. See which major stars he called out below:
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