Why won't our brains let us feel full on liquid food? After all, we spent the first months of our lives living on milk alone. We talk to a man who lived on liquid alone for 30 days, as we explore why adults are ditching the knife and fork in favour of meals in liquid form. We visit a juice and smoothie café in London where a gourmet smoothie can cost as much as a hot roast dinner, and meet a woman who is only too happy to swap her meal for a drink. Sociology and Food expert Anne Murcott, from SOAS, University of London, tells us this trend is all in the marketing, and Richard Mattes of Purdue University explains why our adult brains are not perfectly wired to detect calories in drink form - and takes us on a journey through our digestive system to help us understand how we process liquid food. And a warning about a little known problem that could be hiding in your smoothie, from allergy expert Dr Isabel Skypala. Plus, we talk to the companies making whole meals in a bottle. The CEO of German company Bertrand, Tobias Stöber shares the thinking behind his product. Professor Amy Bentley isn't convinced though. She's from the Department of Food Studies, Nutrition and Public Health at New York University and tells the BBC's Emily Thomas why she doubts the nutritional value of these drinks. (Image: A spilled glass of strawberry smoothies. Credit: Kondor 83/ Thinkstock)