Question about using a thief

[ant8344]

This really sounds like a beginner question, but I cannot figure out how to use the thief to extract samples from the glass carboys.

I have the plastic one pictured here (plastic thing with a handle to the right of the caps)

 

[ant8344]

Here is the pic:

http://www.northernbrewer.com/pics/fullsize/basic-beer.jpg

 

[thedude123]

stick the thief in the beer and then stick your thumb over the whole on the top.