Roll lemons on the counter and cut in half, then juice until you have 3/4 cup fresh lemon juice, straining out seeds and pulp.

Measure and pour 3/4 cup fresh lemon juice, 1 cup cold water, and 1/4 cup granulated sugar into the blender.

Add 3 cups of ice to the blender, packing it loosely into the measuring cup.

Blend on high speed for 60 to 90 seconds until the mixture reaches a thick, slushy consistency.

Check the texture; if it is too chunky, blend for an additional 15 to 20 seconds.

Taste a small amount and adjust sweetness with additional sugar or tartness with more lemon juice, then blend briefly to combine.

Divide the slushie between two tall glasses, insert straws, and serve immediately.

Optional: Chill glasses in the freezer for 10 minutes beforehand to keep the slushie colder longer.