Touch Activated Screens?

I was wondering how the heck touch activated computer screens work. I know some of the early models had light beams crossing in a grid just in front of the screen that were interrupted by the operator’s finger coming close to the screen. But what I’m curious about are the current models that seem to be a normal looking monitor with no visible means of touch detection.

I know, without a doubt, I am about to be given a terrific explanation. Thanks in advance.

(Hey! My second 200th post!)

Scientific American answered this:

The How Stuff Works also explains it.