Watermelon Soda Float

Lime Frozen Yogurt

1 pint store-bought vanilla frozen yogurt, preferably Greek style

1 1/2 tablespoons finely grated lime zest

1 tablespoon fresh lime juice

Watermelon Soda

1/2 medium watermelon, chilled, plus ½ cantaloupe (optional)

1/3 cup sugar

1 tablespoon fresh lime juice

Pinch of kosher salt

1 cup seltzer

Lime Frozen Yogurt

If the yogurt is very hard, refrigerate until firm but scoopable, about 15 minutes. Working quickly, scoop frozen yogurt into a food processor. Add lime zest and juice and pulse just to combine. Return yogurt to same container, cover, and freeze. DO AHEAD: Frozen yogurt can be made 1 week ahead. Keep frozen.

Watermelon Soda

Using a melon baller, scoop out 1 1/2 cups watermelon balls (and cantaloupe balls, if using) into a small bowl. Cover and chill. Cut enough watermelon into 1" pieces to measure 8 cups. Stir sugar and 1/3 cup water in a small bowl until sugar dissolves. Set simple syrup aside.

Purée melon pieces (not balls) in a blender until smooth; strain juice through a fine-mesh sieve into a large measuring cup or pitcher (you should have about 4 cups juice). Discard solids. Whisk in simple syrup, lime juice, and salt. DO AHEAD: Purée can be made 4 hours ahead. Cover and chill.

Add seltzer to watermelon purée. Divide melon balls among tall glasses. Pour watermelon soda over. Top with scoops of frozen lime yogurt. Serve watermelon float with a straw and a long spoon.

Serves 8.