Product hidden as a square gap

What number sits exactly between $37$ and $43$?

1. The midpoint of $37$ and $43$ is $40$, and each is $3$ away.
2. So $37\times 43=(40-3)(40+3)=40^2-3^2$.
3. That is $1600-9$.
4. The product is $1591$.

Answer: $1591$