Commander un pack à l’eau comme un local

What is a “Pac à l’eau” in the south of France?

In Provence, un Pac à l’eau is a glass of bright yellow lemon syrup topped up with cold water. Pac is a brand, made since 1960 by a family distillery in the Vaucluse, and the pattern [syrup] à l’eau is how French cafés sell any syrup drink. Une menthe à l’eau works anywhere in France, but ask for a Pac à l’eau in Paris and you may get a blank look.

The name has nothing to do with lemons: the drink’s creator, Louis Guiot, built it from the initials of his children: Pierre, Annick, Christian and Colette. The speaker orders un pack à l’eau et un espresso, explains that the syrup is diluted (on l’allonge d’eau), and calls it très désaltérant, “very thirst-quenching.” Allonger (“to lengthen”) is the usual word for stretching a drink with water.