[rolph]

pick up your house key, hold it in front of a mirror, look at the key, and the reflection. you should notice the side of key away from mirror is visible, the reflection shows the other side of key .

so the sides of key, and reflection are switched relative to the key.

if you could somhow pluck the reflection from a mirror and try to use it, the left side is right, and right side is left.

when this happens with molecules, there is different parts of the molecules being brought together, leading to alternate interactions, thus different reaction path