Matt Damon was told his daughter would probably "go into shock" after a jellyfish wrapped itself around her chest during a family holiday in Australia earlier this year.
The 47-year-old actor had taken his four girls Alexia, 18, Isabella, 11, Gia, nine, and Stella, seven, on a trip to Byron Bay, New South Wales, with his wife Luciana Barroso in April, but their family vacation ended in tears when his youngest daughter was attacked by a box jellyfish while enjoying a sea day with Matt's friend Chris Hemsworth - who was born and raised Down Under - and his family.
Speaking on 'The Ellen DeGeneres Show,' Matt said: "Australia's famous for having a lot of poisonous things, and I'm telling my kids, 'Don't worry. It's fine.' And Chris is like, 'Oh, you'll be fine. Come on over.' And my daughter, got stung by a jellyfish.
"It was the worst thing. I mean, even Chris said: 'Mate, that's the worst thing I've ever seen in my life.' Like, it was really bad. She was in a little bit of surf, in about a foot or two of water... this thing literally wrapped around her. I could show you pictures that would turn your head. She had these, it looked like red shoelaces around her body."
"[I got stung] a little bit, but not like her. There was enough around her chest that a lot of locals said: 'You should call the paramedics,' because you can go into shock."
The paramedics recommended that Matt bathe Stella in hot water and gently sponge the affected area because the stings, which have enough venom in them to kill the weak, elderly and young, cling to the sponge.
He explained: "They showed up and said, 'Actually, hot water [helps].' Like if you put her in a hot bath and just gently sponge away [the stings]. They stick to you with these little needles, basically. It's really painful."