A lot of students have a hard time reading and understanding rhythms. We're talking quarter notes, eighth notes, whole notes, etc. All that stuff is so crucial to a music student's success, but it can be tricky to grasp when they're first learning what it all means.
I used to struggle to try to have my students memorize all these note names and their beat duration, until one day - out of thin air - I thought of a little visual explanation...