Click the cat to reduce its health. If you want to use the health bar in your own project, backpack the 'Health Bar' sprite and the drag the three custom blocks to every sprite that needs to create a health bar and then delete them from the original health bar sprite. It is now also possible to switch between styles with the 'STYLE' global ('for all sprites') variable! (There's only the 'standard' style included, though. Just remember that there are a few considerations to take in mind when designing your own styles.)
This is a combination of the two different health bars @griffpatch made, but i merged them into one to get the best of both worlds and also did some improvements here and there.