A double health warning is needed on this picture. (1) For pupils: remember that hygiene is very important. (2) For teachers: young children can easily be frightened by the idea of a parasite, so think carefully about how, or whether, to teach the topic.
What are worms (boneless creatures, usually limbless, soft tubes; the garden worm can do the soil a lot of good by introducing air into it)? What is a tapeworm (a parasite that lives off one or more hosts, usually comes from raw or undercooked meat or fish)? Has your dog ever had worms? What did you do to cure it?
What are parasites (creatures or plants that live off another)? Can you think of examples, other than the tapeworm (lice, mistletoe, cuckoos)? What is the term we use when two things live off each other in harmony (a "symbiotic relationship", see the Big Picture of the bird cleaning the impala's ear, June 11 1999)?
How can we avoid getting parasites such as worms (proper hygiene, especially washing your hands, and good food habits, like not eating raw or undercooked meat or fish)? Stress that the worms children are likely to get are usually harmless, if irritating (what's the euphemism for "itchy bum"?), that they can be avoided, and, of course, that doctors can prescribe medicine to get rid of worms.
Ted Wragg is professor of Education at Exeter University