How to prove $ bigcup_ {i in I} A_i times bigcup_ {i in I} B_i = bigcup_ {i in I} (A_i times B_i) $?

Can any one help me prove that? We did not learn in class how to prove that, so I do not even know where to start.

$$ bigcup_ {i in I} A_i times bigcup_ {i in I} B_i = bigcup_ {i in I} (A_i times B_i) $$