What is real?

sometimes we think we know what is real - a color - a shape - an idea - a visualization in our mind - the smile on a friend - the mood we are in - and then we look again and it is not as we thought we remembered it earlier - just a moment ago a week ago - a lifetime ago - things are different - is it our perception? has it changed - have we changed - has the object of our thought changed - has everything changed - is there anything that remains the same in the Universe? Do you know? I don't know ------ anything!