Our naked eyes may not be able to see certain things, but that doesn't make them completely invisible. The Schlieren technique is like an all-seeing-eye method achieved by refracting light on objects before a concave mirror. What it does is that it allows you to see things that would usually be invisible.

This is what the heat of a lighter looks like:

Here's the chill from dry ice.

Watch the video below for more:

No word on whether you can use it to see if your cheating ex had a heart or not.