The Wand tool.
The wand works quite a bit differently than the marquee.
It selects whatever pixel you click on, and any pixel that touches it within a set color range or “tolerance.”
Open an image and click on it with the wand. Now look at your tool settings.
(Remember, just under your menu bar. Or double click your wand if you have a version older than 6.)
Note there is a “tolerance” setting.
This tells your computer to either select that color and anything close, (a higher number,)
or select that color and only pixels touching that are exactly the same color. (A tolerance of 0.)