Bring in a few loaves of bread, jars of peanut butter, and jars of jelly, with a few utensils. Also, lots of paper napkins.
Have the students spend 10-15 minutes writing a 'how to make PB&J sandwiches'. Select volunteers to read their instructions while you follow them as a computer would. Explain that this is how computers work.
Get some bread: grabs entire loaf of bread, uses the entire loaf for following instructions.
Open the loaf of bread: Rips open the entire package.
Open the jar of peanut butter: Fails to rip the lid off.
Spread peanut butter onto bread: Grab a big handful of peanut butter and spread it messily over the bread.
You can demonstrate how a more flexible language allows you to not just stop and crash (ie. Unknown command 'open peanut butter') but can result in far worse results.
You can even get into the different levels of languages by showing the difference between a motion by motion (assembly) instructions vs. one which assumes general sandwhich knowledge (C/C++/etc).