Why is it important for scientists to keep testing a hypothesis even after they've found it to be true?

Explanation:

Because sometimes in the details of an experiment there are details hidden away that no one has thought to exploit.

I like to use Cancer as an example. Most cancers have drugs associated with them that are actually poisons (although  scientists are getting much, much better in reducing side effects).

But the main purpose until recently is an example of survival of the fittest. Think Darwin.  The drug kills off the cancer cells that are most easily killed off. That leaves the slower growing more deadly forms of the cancer behind and they finish the job, because there is nothing to control them, nor is anything competing with them.

Without finding this out, biologists would be at a loss of where to turn. Oncologists repeat the same treatment hundreds of times until the have a feel for how the cancer works. Then they are better equipped to tackle the problem a new.

they are all solids.