If you only had seconds to spot a drowning child, could you do it?
It's a parent's worst nightmare: turning their back on their swimming children for half a second, only to turn back and see they're in desperate trouble.
But that's the speed in which lifeguards operate at in order to save lives.
A powerful video has emerged online of what appears to be a lifeguard whose extremely fast reactions save a child from drowning - in the middle of a crowded swimming pool.
The footage, posted by Lifeguard Rescue to YouTube, shows a wave pool crowded with young children and toy rubber rings.
Just 10 seconds in, a lifeguard is seen diving into the water to save a drowning boy who fell off his rubber ring and was struggling to stay above water.
It appears no one spots the boy's struggle except for the lifeguard, despite a large number of swimmers in the pool.
Many people online praised the lifeguard's swift actions with one person writing: "She is amazing! The fact that she noticed the boy in trouble so quickly, is what saved his life. Things can go wrong in an instant."
"I went back and watched her right when the kid flipped out of the tube. The MOMENT he did, she focused on him and prepared to jump in if needed...and the moment she saw him struggling, she jumped," another said.
It is not known where the footage was filmed, but the video has been viewed more than 18 million times.