Sparkfun Electronics Inventor's Kit - Introducing the Photoresistor

Resistor 10K Ohm 1/4 Watt PTH - 20 pack (Thick Leads)
PRT-14491
Resistor 100 Ohm 1/4 Watt PTH - 20 pack (Thick Leads)
PRT-14493
Introducing the Photoresistor
The photoresistor changes its resistance based on the light to which it is exposed.