Fill your cocktail shaker halfway with ice.
Add the tequila, cherry syrup, Cointreau, and fresh lime juice to the shaker.
Shake for 10–15 seconds, or until the shaker feels cold on the outside.
Pack a Collins glass (or any tall glass) with fresh ice cubes.
Strain the shaken mixture into the prepared glass.
Top with cherryade or club soda.
Stir gently with a bar spoon to combine without knocking out too much fizz.
Add a splash of grenadine if you want a richer red color.
Garnish with a maraschino cherry or a lime wedge and serve immediately.