From hence all beauties, undeceived,
Know one false step is ne'er retrieved,
And be with caution bold
Not all that temps your wandering eyes
And heedless hearts, is lawful prize,
Nor all that glisters gold.

Thats the last verse from a poem called "ode on the Death of a favorite Cat, Drowned in a tub of gold fishes" by Thomas Gray. I like it it is a very good thing to keep in your brain.