a photoresistor, such as RadioShack part #276-1657
a 2N3904 transistor
a 4.7k ohm resistor (coded with yellow-purple-red stripes)
one vibrator motor (This can be scrounged from a cell phone or pager or purchased new at RadioShack—part #273-107.)
a 3-volt CR2032 button cell battery
a CR2032 battery clip, such as RadioShack part #270-009
24-gauge insulated hook-up wire (unnecessary if your battery clip and motor have leads)
two 5/8″ ball bearings