How?
No eating and just drinking water for 80 hours. Starting from the night and then continuing 3 times 24h.

Drink at least
3 liters (13 cups) for a man per day
2.2 liters (9 cups) for a woman per day

Warning
You should not perform this challenge if you have any health issues.
You should talk with a doctor before doing this challenge.
You should drink water constantly and in small quantities so that it won't flush all the minerals.