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To make these you'll need clear flat glass marbles, white glue, access to a decent printer and a sheet full of printed out brains. I just went to the interwebs and found a cartoony picture of a brain then repeated it at about 3/4 of an inch across a page. I decided that the light color wasn't cartoony enough so I colored it pink with a magic marker to make it pop a little bit more.
Then I put a dot of glue in the middle of each brain and placed a flat marble onto it, pressing down to make sure the glue spread out and covered the whole picture. Then I let them dry and trimmed the extra paper off.
I really like the way they turned out, the marble magnifies the image so it looks larger than the print out. That helps with the detail you might lose from making the image so small. Now we can play Zombie Dice in style and I can get rid of those pennies.