On Boxing Day, 2004, Tilly Smith and her family were enjoying an early morning walk along Mai Khao beach in Phuket, Thailand.
The family were on holidays from the United Kingdom and 10-year-old Tilly couldn't help but notice the receding shoreline, and the ocean frothing weirdly at the surface.
The waves weren't coming and going as they normally do at the beach, instead they were just coming.
WATCH: Tilly being interviewed by the UN. Post continues after video.
Two weeks earlier in a geography class in Oxshott, Surrey, Tilly had learnt about tsunamis.
The class had watched a black and white video of a 1946 tsunami in Hawaii, and standing on the shoreline of that beach in Thailand, Tilly was positive the warning signs were the same.
"It was the exact same froth... like you get on a beer. It was sort of sizzling," she told the United Nations Office for Disaster Risk Reduction.