The watermelon has to be chilled, not frozen. Frozen watermelon (or any fruit) do not grind well. Grind watermelon, lemon and sugar together. You can also add a scoop of vanilla ice cream if you are serving immediately.

Sieve the mixture. Mine looks pulpy because the watermelon was almost frozen.

Add soda at the time of serving. Adjust taste with lemon or sugar if required. Add powder sugar if you want to add at this point.

Serve immediately.