"Don't get set into one form, adapt it and build your own, and let it grow, be like water. Empty your mind, be formless, shapeless — like water. Now you put water in a cup, it becomes the cup; You put water into a bottle it becomes the bottle; You put it in a teapot it becomes the teapot. Now water can flow or it can crash. Be water, my friend."
Lesser known variant:
"Flow in the living moment. — We are always in a process of becoming and NOTHING is fixed. Have no rigid system in you, and you'll be flexible to change with the ever changing. OPEN yourself and flow, my friend. Flow in the TOTAL OPENNESS OF THE LIVING MOMENT. If nothing within you stays rigid, outward things will disclose themselves. Moving, be like water. Still, be like a mirror. Respond like an echo."
Unsourced variant: "Be like water making its way through cracks. Do not be assertive, but adjust to the object, and you shall find a way round or through it. If nothing within you stays rigid, outward things will disclose themselves."
Bruce Lee adored water's shapelessness and adaptability as a philosophy for combat and living