Sankhara has the Heart of a Banana Tree!

Suppose, bhikkhus, that a man needing heartwood, seeking heartwood, wandering in search of heartwood, would take a sharp axe and enter a forest. There he would see the trunk of a large plantain tree, straight, fresh, without a fruit-bud core. He would cut it down at the root, cut off the crown, and unroll the coil. As he unrolls the coil, he would not find even softwood, let alone heartwood. A man with good sight would inspect it, ponder it, and carefully investigate it, and it would appear to him to be void, hollow, insubstantial. For what substance could there be in the trunk of a plantain tree? 

So too, bhikkhus, whatever kind of volitional formations there are, whether past, future, or present, internal or external, gross or subtle, inferior or superior, far or near: a bhikkhu inspects them, ponders them, and carefully investigates them. As he investigates them, they appear to him to be void, hollow, insubstantial. For what substance could there be in volitional formations (saṅkhārā)?

A Lump of Foam

Pheṇapiṇḍūpamasutta SN 22.95