From the worksheet: Tangents are where 2 lines just touch each other in a way that causes spatial ambiguity and a slight jarring on our eyes. Most tangents can easily be avoided. Practice looking for these nine tangents in everyday life. As you begin to develop an awareness for them they’ll be much easier to spot in your art/photography, making it a simple task to eliminate them from your compositions early on. 
 
Use the student worksheet to help them indentify the different tangent. The accompanying PDF presentation/images of each tangent uses the same images so you can walk students through them on a screen. Includes teacher's notes and answer key for matching worksheet.
 
On a personal note: I've used this in art and photography. For photography, I have the students take 1-3 pictures for each Tangent to turn in. This is the only time allow them "bad pictures", as I tell them.