My five year old cousin asked a question that completely stumped the
entire family today, including several of us with electrical degrees! 
A few of us make guesses, but we're all pretty sure we're wrong.

On electrical plugs, each of the two thin prongs (not the extra ground
prong) has a circular cutout in it.  For example, each of the thin
prong looks sort of like:

=========
====O====
=========

What is the function of the cutout?

Thanks for asking!

There are actually three reasons why the prongs have holes. 

1. If you take apart an electrical outlet, you'd find bumps on the
contact wipers that match the position of the holes on the plug
prongs. This is done to improve contact, and and insure a firmer grip
-- helping prevent the plug from slipping out of the outlet due to its
own weight.

2. Electrical components may be factory sealed as a safety device. A
plastic band may be passed through the holes in the prongs, rendering
the device unusable unless the band is removed. Some industrial uses,
or construction specs require this type of safety seal.

3. There is a (slight) savings in metal materials. 

Learn more at How Stuff Works:

Why do the two flat prongs on the plugs for electrical appliances have
holes in them?
http://electronics.howstuffworks.com/question301.htm


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plugs -spark prongs


I hope you find this information helpful! 

---larre