Metaphor is an excellent technique for introducing unfamiliar material. It is a proven aid to retention and recall. Through the use of metaphor, students are able to connect new information with something they already know or have experienced, attaching it to existing "schema" in the mind.
The lessons Vulcan's Vocabulary and A Place in Time illustrate the use of metaphor.
Soil
Water
Ice
Rocks
Trees
"The debris avalanche at Mount St. Helens was composed of SWIRT."