Step 1: Picked up this nifty noise maker for about 6 bucks in the toy section of the food store.
Step 2: And I found this little gem in WalMart for about $5.00.
Step 3: Okay. Let's crack open the motion sensor and have a look see...
The Fresnel lens is there to provide a wide field-of view for the motion sensor. Fresnel is pronounced frie-nel, look it up on Wikipedia for more info.