Multiply any two of these numbers together and add 1 and you’ll always get a perfect square:

1 + 1 × 3 = 4
1 + 1 × 8 = 9
1 + 1 × 120 = 121
1 + 3 × 8 = 25
1 + 3 × 120 = 361
1 + 8 × 120 = 961