How do they do this?

Sheldon said:

It has to be something to do with a double blind removal similar to a old magic trick I use to do but... Link Removed

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NDS said:

It's very simple if you look. The numbers are grouped by color--5 numbers per color.

You click on the color of the number; then that group is separated into 5 groups again, this time only one of the numbers of that color is in each of the new groups. When you pick your number again, there is only one number that was that color and in that second group. The rest is fluff.

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