“I consider myself a fairly water-conscious person,” says the average American, sipping on a venti iced coffee while dipping his toes in an Olympic-sized pool, spritzing himself with Evian. “I probably just use a few gallons a day,” he continues, stepping out of a 45-minute shower. “By the way — have I told you about my toilet that flushes automatically every 20 minutes, just to make sure it’s consistently pristine?”

Just kidding — it’s not quite that bad. But, according to a recent study in the journal Proceedings of the National Academy of Sciences, the average American consumes twice as much water as she thinks she does. Furthermore, we Americans are not quite sure which practices are the most water-intensive. As it turns out, the Olympic-sized pool isn’t the biggest concern — 70 percent of personal water use occurs within the home, according to a 2005 EPA study. And the biggest culprit under the roof? Toilet-flushing, accounting for 27 percent of all indoor water use.

Perhaps most troubling, Americans overwhelmingly believe that changing their habits, as opposed to improving the efficiency of their plumbing, is the most effective way to cut down on water consumption. Seventy-six percent of those surveyed said curtailment methods, such as flushing less frequently, are the best way to reduce water use. Only 10 percent chose more preventative measures, such as installing new toilets that use just 1.6 gallons per flush in lieu of old toilets that use five to six gallons.

The concept of “embodied water” — also known as virtual water, or the amount of water required to produce a certain quantity of food — was also relatively unknown to the study subjects. A pound of coffee, for example, has a water footprint of 2,264 gallons.

Study author Shahzeen Attari, an assistant professor at Indiana University, reminds us that, contrary to popular belief, we can’t count on the unlimited availability of freshwater. “Most Americans assume that water supply is both reliable and plentiful. However, research has shown that with climate change water supply will become more variable due to salinization of ground water and increased variability in precipitation.”

California’s certainly learning that the hard way right now.

To sum up: Water is involved in pretty much everything you do, and its supply is limited because the planet is being destroyed. Keep that in mind next time you’re in the bathroom.