The temperature sensor is located here on the Circuit Playground Bluefruit:
If it senses below 20° C, the "cool.gif" image will play on the TFT. Between 20°-24° C, the "neutral.gif" image will play, and above 24° C, you'll see the "hot.gif" image.
It changes with body heat or with your breath (with a little patience). Or hold it near the fire or in the freezer to make it change dramatically!
To charge the necklace, gently pull the battery cable out of the socket and plug it into your USB charger.