Thermistors are pretty cool little items. They convert heat into resistance. By having the temperature available as resistance, you can use the value to control other things like circuits and programs. Photo cells do the same thing with light, and they are in lots of common devices from night lights to dimmers on clock radios.
How could you use a thermistor with your shiny new Arduino? What could you measure with a thermistor? Have you got any videos like this where somebody demonstrates an interesting device? This video came from a summer youth program in Boston. Does your community have a similar opportunity for teenagers to learn incredible things and work on neat projects? Show your ideas in the comments, and iinclude your photos and video in the Make Flickr pool.