We match four pieces of talk with four short words. Only then the rocks give way. Click to see them turning.
In school speak, the short words are prepositions. They may tell where something is. We use our language intuitions.
The four pieces of talk:
I am thinking.
I have thought.
I have been thinking.
The four short words:
TO ON AT IN
I have done one for you:
TO, I have thought.
We think if the short words would match different, if we said,
I thought. / I will think.
I was thinking. / I will be thinking.
I had thought. / I will have thought.
I had been thinking. / I will have been thinking.