A small number trick
Brown’s Criterion
Think of a number from 1 to 31. Then choose every list that contains it. I’ll try to guess what’s on your mind.
Choose each list that includes your number.
How it works
A little binary arithmetic.
Every number can be expressed as a unique combination of powers of two. Each list represents one of those powers; selecting the lists that contain your number gives the answer away.
For example, 25 is 1 + 8 + 16. It appears only in the lists headed by 1, 8, and 16, so adding those headings reveals 25.
