The Fault in our Stars heartthrob turned 21 on Saturday (14Mar15), and he urged fans to donate to his favourite organisation as a birthday gift.

Elgort launched a contest on to raise funds for The Thirst Project, which provides water to those in need around the world.

In a message posted on the campaign website, Elgort writes, "For my birthday this year, I wanted to do something different and special... and so here I am, to raise money to help provide safe, clean drinking water and proper sanitation to people that only have access to dirty, disease-ridden water.

"Every dollar helps, so whatever you can do, please do because it all adds up - and the goal here is for us to make a difference together!"

Just one day after the campaign went live, one lucky fan splashed out $20,000 (£12,500) on a trip for two to meet Elgort for lunch, while another handed over $100,000 (£62,500) to land the actor as a prom date.

Elgort took to on Sunday (15Mar15) to thank all his fans for their generous donations, writing, "OMG (oh my God) guys! This is amazing!!! I'm so touched. People are donating like crazy!!! Wow. Thank you this is the best present ever."