Kids see those apostrophes all the time, but do they really know what they're for and how to use them correctly? I was surprised how many of my 8th graders said they had never had any formal instruction in the concept (and it sure showed in their writing).
Here's (see what I did there?) a bright poster that summarizes the primary contractions of pronouns and linking/helping verbs. You could print it out in B&W 8 1/2 x 11 for students to keep in their notebooks, print color copies to put up in your room, or do as I did, and take it to an office supply store and have a big 16" x 24" poster printed for about $20.00.
Great for your unit on using apostrophes, possessive vs. contraction, general grammar, or proofreading.