To realise that things we habitually say and do and think are actually wrong. You walk down the street, looking this way and that. You think you're a geezer. That woman's pants are too tight. You look over the railway bridge and you think somebody said something interesting.
Or rather you have no idea whether these things are right or not. Your consciousness is so thin as to barely exist. You are a watchman who tries to make sentences span the disappearance into nothing of the past and the nothing that comprises the future, to carry them over the line between those two nothings, an imaginary concept called the present. When in reality there is nothing at all because the past is always gone, the future is never here and there is no interval between them, no space where something can exist that is neither past nor future. And how can you carry something from nowhere to nowhere? What can persist across the line between two empty zones with no gap between them? But something does, apparently.
But there is no other copy in a different zone, is there?, only this one that keeps changing its configuration. Everything is still here only in different arrangements. Something about change. If nothing changed, time would perhaps not exist. There would be nothing to count it by. Counting could not occur because counting itself involves change. To count is to see different things. If you see different things, your vision is changing. It takes time to exist. No time, no time to exist. It takes space to exist. No space, nowhere to exist, no existence. And so what we have is existence because equally without the existence of something, and its somewhere, there can be no time and no space. What could time possibly mean without anything in existence? And without time, no space, no time for space to exist. We might just as well call time/space existence.
There is no past somewhere else. There is no future waiting in the wings. Only the perpetually changing present. In existence. Tumbling.
Time is not a separate thing with attributes. It's a measure of things. You can't get a bottle of time, just as you can't get a bottle of length. Things have length and things likewise have time. Therefore time is a property of things, the fourth dimension. We are not equipped to see the fourth dimension. If we could, we could turn our attention through past and future, just like we look at a huge ocean liner in port, turning to the prow, looking along the side with all its portholes, scanning along and looking down to the stern. We could look towards how it was last week, when there was no ship here, scan along as it sailed this way until it arrived here where we are now and by continuing to turn forward in time, we might see it sailing away again. But we can't!
Yet in a sense we can - by staying put and waiting. We get to see the ship arriving and departing. So perhaps what we see is whatever intersects with our own time. We see all of our time in a way we can't see all of other things' or people's time. We only see them when their time intersects with ours.
It's time that is passing through us. It changes and rearranges all, while we never move any more than a tree moves, or a pebble in a stream.