Can a computer read your thoughts? Seems far fetched, but lets give it a shot. Think of a number between 1 and 21, then choose the column that contains it. You will have to make this selection a few times, and the computer might be able to infer the number you are thinking.
| A | B | C |
|---|---|---|
| 1 | 8 | 15 |
| 2 | 9 | 16 |
| 3 | 10 | 17 |
| 4 | 11 | 18 |
| 5 | 12 | 19 |
| 6 | 13 | 20 |
| 7 | 14 | 21 |