I remember the first time I noticed that I could guess correctly which fingertip a friend shining a tiny red pen laser was shining it on, just from minuscule te

I remember the first time I noticed that I could guess correctly which fingertip a friend shining a tiny red pen laser was shining it on, just from minuscule temperature differences. That you can feel artificial lighting as heat (the actual light, demonstrated as it was a laser some distance away) at such a low power level (<5mW) was pretty surprising to me.