With lots of tennis balls, lots of cardboard, and lots of time, something was bound to happen and this is what came out. I was getting rid of my old books, so I ended up with very few that kept falling over because they didn't span the entire shelf length and so I made a bookend.
Step 1: Cut Cardboard
Cut the cardboard to the length you want it to fit on your shelf and hot glue it together. I went with three pieces of cardboard to try to make it more supportive.
Step 2: Split the Tennis Ball
I used a pocket knife to split the tennis ball and put a ring of hot glue on the circumference. Press it to the cardboard and add a ring of hot glue connecting the ball to the cardboard.
Step 3: Size It Up
Because I had so many tennis balls (and I didn't exactly cut them in half) I used two larger ones on the same board and two smaller ones to make it closer in size. This way the books will be touching both halves instead of just one if it were lopsided.
Step 4: Bookends in Action
You can put the bookends anyway you want it (that's the way you need it), anyway you want it! (Guitar music here). I personally am fond of putting the cardboard next to the books and the tennis balls facing the bookcase, but it is totally up to you. Sorry about the last picture, it would be correctly rotated on my computer and phone but didn't show up correctly here.