Astronaut Ice Cream is real ice cream which has been freeze-dried. It doesn't need to be kept cold - and it lasts forever! (Or near enough - the sell-by date is typically about 2 years from date of purchase.)
For a really memorable space science lesson, order some Astronaut Ice Cream and let the pupils try it. (One packet is sufficient for about 20 people to taste - although some people can eat a whole pack on their own!)
Ask them to think about how Astronaut Ice Cream differs from ordinary ice cream. It isn't cold, it doesn't need to be kept in a freezer, has a very different texture, and it weighs a lot less (i.e. has less mass) than an equivalent quantity of ordinary ice cream. But it is nice and creamy, just like real ice cream.
Price:   £2.50 per pack
Warning: May not be suitable for people with certain food allergies.