Tell us about yourself!
Tools and Materials for Laser Cutting
Laser Cutting Class
Transforming your world with Tinkercad
LEDs and Lighting Class
Or fill it with ketchup/(red food coloring liquid) for those in film/video for a cheap fake-blood packet.
On second thought, you've got a great idea slydersnyder, you could easily fill different bubbles with different colors and it would be a good way to do a rough sight-in of an air rifle on up. It would be easier to see from a distance or using binoculars to see where the shot hit the target.