BELLEVUE, Wash. -- Researchers at Expedia's Innovation Lab use sophisticated science to understand how and why people pick vacations.

The overall message: you don't know what you think.

Expedia's researchers use eye-tracking and facial-movement technology to measure what people look at and why.

Sensors detect slight changes in facial expressions that suggest feelings of happiness, frustration, or anxiety.

Behind a two-day mirror, Richard Hazlett, a Ph.D. in psychology, studies graphs and charts and probes testers to see what part of Expedia's online experience stirred up certain emotions.

"Think of it as a lie detector for your emotional state," said Scott Jones, who oversees Expedia's user experience. "You say customers surely know if they're happy or sad. No. Customers and travelers in general, humans in general, have very delicate, very detailed thought processes."

Expedia determined people like the photos on its website because they smiled more in the lab when looking at pictures of hotels and other travel destinations.

As a result, Expedia beefed up photography and suggested that hotels provide better and brighter pictures.