Concrete
https://www.google.com/search?q=concrete&rlz=1C9BKJA_enUS590US590&oq=concrete&aqs=chrome..69i57j0j5j0j69i61.2700j0j7&hl=en-US&sourceid=chrome-mobile&ie=UTF-8
Abstract
https://www.google.com/search?q=abstract&rlz=1C9BKJA_enUS590US590&oq=abstract&aqs=chrome..69i57j0j5j0.2667j0j7&hl=en-US&sourceid=chrome-mobile&ie=UTF-8
I know very well what both terms mean. The ability of a bird to fly is NOT abstract you can get thousands of images and videos with a simple google search. Particle physics and asexual reproduction are also observable hence NOT abstract.
Once again if you wish to teach something a working knowledge is a good thing to possess before making yourself look foolish AGAIN.