A few hours in advance (or even the day before), steep a green tea bag in some freshly boiled water. Set it aside to cool to room temperature and then stick it in the fridge to chill (adding a few ice cubes always helps!).
Once your green tea is chilled, add 1 1/2 cups of it to your blender. Add the chilled watermelon cubes and lemon juice.
Blend on high speed for 45 seconds to 1 minute, or until the mixture is completely blended.
Pour into a glasses and serve with a slice of lemon!