The Cut the Cake Puzzle

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The Solution

Kudos to all those smart people
for sending in the correct solution.

Here goes:

Use the first two cuts to cut an 'X' in the top
of the cake. Now you have four pieces. Make the
third cut horizontal, which will divide the four
pieces into eight. Think of a two by two by two
Rubik's cube. There's four pieces on the top tier
and four more just underneath it.

The Original Puzzle

You must cut a birthday cake into exactly eight
pieces, but you're only allowed to make three

straight cuts.
You can't move pieces of the cake as you cut.
How can you do it?