It’s only a matter of time before Earth’s growing population faces a critical shortage of potable water. Luckily, science has a foolproof solution. A process called reverse osmosis can convert wastewater into H20 that’s as pure as the distilled stuff—even cleaner than what we usually drink. There’s just one problem: persuading people to drink liquid that used to be urine. After all, humans tend to pooh-pooh (pun intended) anything they find disgusting.

That repulsion response evolved to help us avoid ingesting things that are potentially dangerous. (Rotten food grosses us out for a reason.) But if humanity is going to survive, we may have to learn to overcome the ick factor.

University of Pennsylvania professor Paul Rozin (nicknamed the King of Disgust) has been examining this issue for decades. In a 1986 study, he asked students to drink a cup of juice and rate it. Once that was done he put a cockroach into a cup of the same juice, stirred it around a bit before removing it, and asked them to drink. The bug was dead, and he assured them that it had been sterilized. But not surprisingly, almost no one wanted a sip. In their mind, the cup was contaminated. So Rozin took a brand-new, clean cup and poured fresh, uncontaminated juice into it. This last cup of juice scored lower ratings. The repulsion was so intense that it tainted unrelated objects.