Chair worry

Once I was walking back with 2 friends from school to home. I was about 17. 
These guys were both smart and they definitely knew it. It was as if they knew that school/college wasn’t for them.
In the previous few weeks I had worked out that words, and words attached to objects have been made by us and that we have been told that the way of it. So I realised that the chairs could be named anything rather than chair, this blew my mind as basically told me that everything that I thought was concrete was not. I decided on this walk home to mention this to my friends. 
I think I remember saying ‘a chair is not a chair it’s just a thing’
I felt vulnerable saying this and went to them for support and for assurances, instead they both laughed at me and thought that I was being so stupid/obvious. It’s as if they were telling me that they had got to this step about 3 years ago. 
This moment stays with me to this day as