How are lasers used to determine the distance between earth and the moon?

-- A very short laser pulse is shot from the Earth toward the Moon. 

-- It reflects off of the moon's surface.

-- It's detected when it returns to Earth. 

-- The time is measured between when the pulse is fired and
when it's detected. 
(The round-trip time to-and-from the Moon is about 2.56 seconds.)

-- Since we know the speed of light very precisely and accurately,
the round-trip time tells us the length of double-the-distance.

Why that works with a laser and not with a flashlight:

-- Every beam of light spreads out after it leaves the source.

-- If the beam spreads only 1 degree, then it would be wider than
the Moon when it got there, and it would be about as wide as the
Earth when it returned. The whole thing would never work.

-- A laser beam spreads very very very little, and PLUS, when it's used
to measure the distance to the moon, they shoot it through a telescope,
which concentrates it into a beam that spreads even less.

Its like radar they fire a laser and the time it takes for the laser to hit the moon and return can be calculated to find the distance from Earth the the Moon.