Thanissaro Bhikkhu

But when you see the bones of people, your perception labels them.

That's a person's skeleton.

That's a person's skull.

If there are a lot of them, they can really scare you.

When you see the picture of a skeleton or of anything which shows the inconstancy and non-selfness of the body,

If you don't see clear through it, you'll get stuck at the level of skeleton and bones.

Actually, there are no bones at all.

They're empty.

Nothing but elements.

You have to penetrate into the bones so that they're elements.

Otherwise you'll get stuck at the level of skeleton.

And since you haven't seen through it, it can make you distressed and upset.

This shows that you haven't penetrated into the Dharma.

You're stuck at the outer shell because you haven't analyzed things into their elements.

When days and nights pass by, they're not the only things that pass by.

The body constantly decays and falls apart too.

The body decays bit by bit, but we don't realise it.

Only after it's decayed a lot, when the hair has gone grey and the teeth fall out,

do we realize that it's old?

This is knowledge on a crude and really blatant level.