[Herald1360]

Slightly more mathematically......

P = V * I

for a resistor,

I = V/R

so P = V * V/R which is Vsquared over R.

Similarly

V = I * R

so P = (I * R) * I which is Isquared times R.

A couple of useful visual aids are triangles

..V...
I * R

..P
I * V

Cover up the value you want and the formula to get it is left visible.