What is stereoisomers and chirality?
Got a friend? Of course you do!
Try to shake his left hand with your right hand. Will it work? Why not?
This is an example and manifestation of chirality, handedness. In general, a carbon atom that is bound to four different groups,
How to remember this? Make models and convince yourself that these propositions are true. There should be many other problems on this site that deal with this issue.
PS Last year I had a class with identical twin brothers; they were nice young lads. Anyway they kindly demonstrated these principles of chirality to the rest of the class; i.e they shook hands in the regular fashion, and then pulled all sorts of contortions trying to shake right and left hands. And of course I asked the class why they could not shake right and left hands.