Put the sour cherries in a jar.
Pour in the sugar.
Smash the fruit with a wooden spoon, potato masher, or rolling pin.
Park the jar in the fridge overnight.
Check the jar the next day; if the sugar is not dissolved, shake the jar and put it back in the fridge.
After a day or so, you should have a jar full of fruit and syrup.
You can strain the fruit from the syrup or add the fruit to your drink.
