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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.
The rind of a fruit is the hard outer layer that you can't eat. Fruits like watermelon and oranges have rinds.