In the early 1900s, a horse by the name of Clever Hans appeared to be able to do math and tell time. They asked the horse something like, "What's 3 = 7," and he tapped his hoof ten times.
I'll bet that was front page news. Everyone was talking about that horse at one point. It was front page news, along with some celebrity scandal. Find a 1907 newspaper, and I'll bet you'll see what I'm talking about. The first story will be about Hans, and the next one will be about the 1907 version of Kim Kardashian.
But of course, people later discovered that Hans wasn't actually doing math. He was merely reading the body language of the person asking him questions. Whenever Hans reached the correct number of taps, the person's body language changed--and that prompted Hans to stop tapping. In other words, people unknowingly sent him information through their body language--and Hans was able to pick up on it.
The Clever Hans effect occurs when we gain information from somoeone through her body language, expressions, clothing, etc., only she doesn't realize it--and then we present that information to her. We ourselves might not be aware of what we're picking up on, either. For instance, we might not realize that we're noting someone's body language.