Make a simple syrup: combine filtered water and sugar in a small microwave safe bowl. Microwave for about a minute, or until water simmers and the sugar dissolves. Let cool completely. (This simple syrup will keep in the refigerator for about a month.)
Rub the lime slice around the edge of a tall glass, then dip the rim into a small plate of salt. Fill the glass half full with ice cubes.
Pour the grapefruit juice, tequila, lime juice, and 1 tablespoon of simple syrup into the glass. Top with sparkling water and stir to mix all ingredients together. Garnish with a lime slice.
Notes
This recipe makes 1 serving, but can easily be increased to serve however many you'd like. You can prep all of the ingredients ahead of time, just don't add the sparkling water until ready to serve.
Fresh squeezed or bottled grapefruit juice work great here. Just be sure to get the unsweetened variety.
Silver tequila is a milder variety that works well in palomas. Feel free to use whatever tequila you have on hand.
Leftover simple syrup is great to have on hand to sweeten other cocktails or stir it into your iced tea.