[match tricks six] try before you check

Now this one doesn't easily spring to mind. You need to think laterally but it does make sense:

First arrange 12 matches to make 3 squares.

Take away 1 match, rearrange 2 matches and leave one.

Check the solution here.

You might like to try these too:

Matchtricks 1
Matchtricks 2
Matchtricks 3
Matchtricks 4
Matchtricks 5