What should be on each side of the dice?

1 as opposed to 6

2 is the opposite of 5

Opposite to 3 is 4.

The corresponding opposite sum is 7,

The sum of the six faces is: (1+6) × 6 ÷ 2 = 2 1.

2 1 is divided into 3 pairs, and the sum of each pair is 7.

Good wishes and goodbye.