The Clinic and the Key

so me and Gracia were assigned to make a mural for the clinic. We made a few designs and the nurse liked this one. Origionally it was only the thing that said clinic with the stethiscope (i'm not really sure who drew the origional design), but she wanted it to have CHS somewhere on it, so we remade it with CHS (for Chantilly High School). Eventually this will be on the wall in the nurses office:



In art class, they gave us a Skeleton key and told us to use it as inspiration for a drawing or add stuff to the key and make it jewelry or something, we can paint it...etc... Not sure what I'm going to do with it. Why's it called a Skeleton Key anyway?