I like thinking of it like the Olympic rings. The blue ring(all the way to the left) doesn't need to know anything about the red ring(all the way to the right),

I like thinking of it like the Olympic rings. The blue ring(all the way to the left) doesn't need to know anything about the red ring(all the way to the right), but should absolutely know the yellow ring(second ring from the left), and possibly bits of the black ring(middle ring). The leakier the abstraction, the more you need to reach.