Another algebraic manipulation skill that I want to share with students today:
Rewrite $a^2c^2+b^2c^2+a^2d^2+b^2d^2$ as the sum of two squares.
The vast majority students would say, hey! Isn't it obvious that we can factor out $a^2$ in $a^2c^2+a^2d^2$ and $b^2$ in $b^2c^2+b^2d^2$ such that we get: