Water

Blue at daytime, black at night,
Reflects any kind of light.
Water can sustain all life,
Water can't be cut by a knife,
Whilst by itself can fool your sight.

Firefighters use it when on a truss,
Drivers use it to clean their bus.
Is used by gardeners to feed their plants,
Is used by surgeons for heart transplants,
Water is more than half of us.