How do I capture part of the screen on the Mac

Problem: I want to take a screen shot of part of the screen on my Mac. I want to capture only a part of what the monitor on my Mac is showing.

Solution:

Command key - Shift key - 4 key

Details:

1. Hold down the command key. Hold down the shift key. Now press the 4 key. Let go of the keys.
2. Your cursor will change shape. Now use it to draw a rectangle around whatever you want to capture. To draw the rectangle, hold down the left mouse button. If you're using an Apple mouse, you only have one mouse button, so hold that down. 
3. When you let go of the mouse button, a file containing the captured image will appear on your desktop. It will be called Screen Shot <today's date> <today's time down to the second>

And there you go, problem solved!

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The icon that shows up on your desktop will look something like this.

The name of the file got shortened; it's only showing me the beginning of the name and the very end. I didn't take the screenshot at 3 AM. I took it at 9:15 am and 3 seconds. It's confusing.

Here's a link to a page that tells you how to change the default filenames for screen captures. I haven't tested out this method, so I can't vouch for it.

What I did was go change the clock on my screen to show the seconds. Click here to learn how to change your Mac's clock so that it shows seconds.