Stephen Hawking's Alt+F4 mishap

Listen up, buttercup. This joke implies Stephen Hawking, who was known for using computers to communicate, died because he accidentally pressed Alt+F4. Alt+F4 is a keyboard shortcut that closes the current application window in Windows. The joke is funny because it suggests he accidentally shut down his 'application,' which is a silly way to think about death. You wouldn't get it, but I had to explain.