Here's a neat little trick to try on your friends.

What you do:

Ask someone to add up the following sequence of numbers, and tell them to tell you the total out loud.

You say 1000, they should say 1000.
You say 40, they should say 1040.
You say 1000, they should say 2040.
You say 30, they should say 2070.
You say 1000, they should say 3070.
You say 20, they should say 3090.
You say 1000, they should say 4090.
You say 10, they should say 4100, but most people will say 5000.

Try it on someone. Now find out how it works.

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