How to make a paper airplane curve down and fly upside down?

Not sure if this is a serious question but the trick is to make minor adjustments to the back of the paper plane's wings.

To make the plane to nose down and then fly upside down, bend down the back of the wing. Keep the adjustment small - a little bend goes a long way.

There are instructions with pictures and videos online to show you how.

snell's law

refractve index = sin incidence / sin refraction