Why couldn't Bob hang himself?
Because he had no arms to tie a knot. :'-)
Listen up, numbskull. The joke is that Bob can't off himself because he LACKS ARMS. Can you imagine being so dumb that you lack arms? Well, maybe you don't have to. I bet you still don't get it, so here's a fun fact for you: Bears are known to use tools to catch food. You, on the other hand, are a tool.