A screencast, which is a digital recording of a computer’s screen, can be helpful when trying to demonstrate or share something from your computer.  To make a screencast, simply open QuickTime and go to File > New Screen Recording, and you’ll see the following pop up on your screen:

If it’s the first time you’re using this function, you’ll need to select an audio option; otherwise, the recording will be without sound.  To choose your computer’s built-in microphone, use the drop down box as follows and choose “built-in input”:

To start, click on the red recording button, and you will be asked to indicate whether you’re recording the entire screen or just a portion of it.  Keep in mind that no matter what you record, the little black box you see pictured above will not appear in the screencast.