Can you help explain this quote from a tale of two cities book 4?

It makes fun of what all the people fear, the guillotine. Sometimes the only relief from horror or fear is humor. Can you see how the guillotine cures headaches -- you have no head--. Your hair won't turn grey because you'll be dead. If you look through the window that means that your head is about to be chopped off and your head falling off is being compared to a sneeze.