Thank you to to Connor and Cameron for bringing in their pet snails to show us.  We had a good look at them and learnt that :

The body of the snail is long, moist and slimy. It has a shell to protect the soft body. When the snail is disturbed, it simply withdraws or pulls itself back into its shell. The snail also retreats into its shell and seals the entrance in dry weather to protect its body from drying up. A snail is most active at night and on cloudy days. It does not like the sunshine very much. During very cold weather or winter, it hibernates in the ground.

Sounds as though snails are very clever !