In the quest to find foods and drinks that will help flatten abs, there are a lot of big claims made by manufacturers and few real results.

Many of us have become so used to seeing advertisements for the most complicated or expensive metabolism-boosting beverages imaginable that we’ve forgotten that simple recipes can be some of the most effective.

There is one drink you should have right when you wake up if you want a flat stomach. It just so happens to be super cheap — or, depending on where you live, completely free. And the best news of all is that you’re probably already drinking it.

It doesn’t get more pure, simple, or affordable than this. Water isn’t fancy or naturally flavorful, but it’s readily available and has science to back up the fact that, yes, it really can help you get flatter abs — by boosting your metabolism through a bodily process called thermogenesis.

One study published in the Journal of Endocrinology & Metabolism found that consuming 500 ml of water (which is a little more than two cups) can increase your metabolism by 30 percent.

“Within 10 minutes after drinking water, energy expenditure started to increase,” according to the study. “Sixty minutes after drinking water, energy expenditure increased 30% in men and 30% in women.”

Aside from the major fact that water increases your metabolism and actually makes it easier for you to lose weight and burn fat, it’s one of very few beverages that contains absolutely no calories.

The simple act of replacing one or two (or all) sugary drinks with water helps you burn more calories, while suppressing your appetite to keep you from overeating. Water also flushes out toxins in your body and gives your digestive system a boost to help you eliminate waste faster and more efficiently.

So, how much water should you drink? The daily water requirement is (on average) about two liters each day, which works out to eight, eight-ounce glasses.

But that amount can change depending on several factors. If you are breastfeeding, you’ll need to drink more water. If you work out and sweat a lot, your body will likely let you know that it requires a few additional glasses. Your best bet is to use the daily water requirement as a baseline and then adjust it when needed.

Many experts will tell you that drinking water first thing in the morning (with or without lemon) is the perfect way to start your day and slowly break your fast from the night before. And it’s such a common practice in places like Japan that it’s been given a name: Japanese Water Therapy.

If you choose to try this “therapy,” you’ll need to wake and drink about four glasses of lukewarm or room temperature water before you eat or drink anything else. Take a 45-minute break before you eat breakfast. Then continue your day the way you normally would — but don’t forget to drink water between and with meals.

If fans of this therapy are to be believed, it can help keep weight off, increase stamina, burn fat, and prevent sickness. You won’t have to send away for expensive boutique drinks or waste hours preparing your beverages — so it’s definitely worth a try.