If you eat the food and drink the water, you will feel full for a couple of hours before hunger kicks in. But if you blend the food with the water - to make soup - you will stay hunger-free for much longer, and less likely to snack through the afternoon.
How can blending the food into soup make such a difference? The answer lies in the stomach. Scientists have used ultrasound and MRI scans of people's stomachs to investigate what happens after eating solid-food-plus-water meals compared with the same food made into soup.
After you eat a meal, the pyloric sphincter valve at the bottom of your stomach holds food back so that the digestive juices can get to work.
Water, however, passes straight through the sphincter to your intestines, so drinking water does not contribute to "filling you up".
When you eat the same meal as a soup, the whole mixture remains in the stomach, because the water and food are blended together. The scientists' scans confirm that the stomach stays fuller for longer, staving off those hunger pangs.
The key to this low-tech weapon against hunger is a hormone called ghrelin. It is one of the major players in the body's appetite system.