If u=|M| and v=|N|, that is, u and v are the number of elements in their respective sets, then we can represent M={ m₁, m₂, m₃, ..., mᵤ } and N={ n₁, n₂, n₃, ..., nᵥ }.
M×N={ (m₁,n₁), (m₁,n₂), (m₁,n₃), ..., (m₂,n₁), (m₂,n₂), (m₂,n₃), ..., (mᵤ,nᵥ) }.
So for element m₁ we have v products, therefore for all u elements of set M, we have a total of uv products in M×N, and so |M×N|=uv=|M||N|.