premise: How to check engine vacuum hoses<br>Check each hose for bulges and swelling. If a hose feels soft or spongy when you touch it, this can indicate damage caused by your car's oil. [substeps] A swollen hose that is cracked and hard is often caused by heat damage.
hypothesis: If a hose is hard its from damage to the hose
reason: The last sentence of the context states that a hose becomes hard when it's been damaged by heat.