You know the feeling: you step off the airport walkway and your body seems to be faling forward over your toes. Or you get off the treadmill and the drinking fountain seems to rush toward you. Or the classic visual illusion: after looking at a waterfall for 30 seconds a stationary boulder seems to drift skywards. Why?

It turns out a very basic mechanism of your brain is to blame: "normal" is what your brain says it is. Here's a deeper look:

When you first look at or experience a stimulus, the neurons that recognize it get excited. They spring into action, processing the new information and forcing it into your consciousness: wow, look at all that falling water! Then the neurons get bored. The stimulus gets blasé and drifts into the background.

And your neurons adjust their expected baseline—falling water starts to look stationary—and you start to interpret things in relation to this baseline. And if falling is stationary, then stationary is up! So when you look from a waterfall to a rock, your baseline is off and the rock appears to levitate until your neurons readjust.