I knew someone years ago - I'll call him Pieter - who was very serious about alternative medicine and even practiced it. He believed in biological energy that emanates from people, and which (if you have the right abilities) you can detect and use to determine the state of their health. One evening I invited him to a drinks at my house, where he met another friend who I will call Francoise. Afterwards, my wife said she thought Francoise was pregnant, and indeed two weeks later Francoise announced that she was.
When I told this to Pieter, he was intrigued. How had my wife known this, given that Francoise had said nothing at the time? (Perhaps he was also a little peeved, given that his own energy detectors had clearly failed).
How had my wife guessed? Francoise had invariably accepted the offer of a glass of wine before this. This time, she only wanted water.