Objective: How do you scare your sister with green army men?

Which of the following solutions is more sound in terms of naive physics reasoning?

Options:
(a). Take one and gently cut it's head off. Fill the cavity with ketchup and glue the head back on. Line them up in your backyard and shoot them with a BB gun..
(b). Take one and gently cut it's head off. Fill the cavity with mayonnaise and glue the head back on. Line them up in your backyard and shoot them with a BB gun..
(a).