Something is behind you if you cannot see it - your back is facing it rather than your front. The horse in the picture is viewed from behind.
You grind something by breaking parts of it off with friction. Metal or wood can be ground down by running a very rough surface over it, such as a grinding disc or sandpaper. Grinding will gradually change the shape of the thing being ground as the outer layer is removed.
You grind herbs and spices in a mortar and pestle by crushing them against the inside of the bowl.
Mankind is a word used to describe all of the people of the world.
The rind of a fruit is the hard outer layer that you can't eat. Fruits like watermelon and oranges have rinds.











