Use the numbers $1,2,3,4,5,6$ to fill a $6 \times 6$ grid, as shown in the figure on the right, with each small square filled with one of these numbers. The sum of the four numbers in each $2 \times 2$ square is called the "indicator number" of this $2 \times 2$ square. Can a filling method be given such that any two "indicator numbers" are different? If yes, please provide an example; if not, please explain the reason.
26. Answer: No

Analysis: The sum of the four numbers in each $2 \times 2$ square is at most 24 and at least 4, giving a total of 21 different values from 4 to 24. However, in a $6 \times 6$ grid, there are 25 different $2 \times 2$ squares, meaning there are 25 "indicator numbers." Since $25 > 21$, according to the pigeonhole principle, there must be at least two "indicator numbers" that are the same.