The mind cannot grasp impermanence. Knowing it all at once destroys the mind.
When insight into impermanence suddenly destroys the mind, the mind's energy is unbound from minding. When the mind's energy is unbound from minding, it annihilates every last trace of "mind" along with all its objects. "Suddenly I was ruined and homeless." (Joshu)
A mallet hits a bell. The air resounds for miles. A crow screams in the big pine. Ants crawl out of a hole. Yellowjackets swarm around a pile of sand. This morning it was cool. This afternoon it is hot. Who is hot or cool? Someone is thirsty. Drink a gulp of cold water. What's the problem?
If you know this one thing, you know all things. What is it that instantly knows? For this question there is no answer, because there is no possible object or subject of the question, and therefore no need to ever ask it.