Ice. Caffeine. Bubbles. Together, they form an ideal hot-weather drink — and, no, we're not talking about a syrupy Coca-Cola.

Meet the coffee tonic (we call it a C&T), a specialty drink that turns your favorite caffeinated beverage into your favorite fizzy thirst-quencher — particularly for those who love bitterness.

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The drink is made most often with a double shot of espresso poured into a glass of tonic water over ice.

That's the case at Double Knot, which has been serving the summery option for about two years, using a single-origin bean from Brazil and a house tonic. Together, the liquid infusion creates a slightly citrusy iced beverage with a thin layer of creamy espresso foam at the top and a double layer of bitter complexity underneath.

"We wanted some refreshing and delicious espresso-based drinks that stayed true and honest to the coffee without totally masking it in syrups and sugar," said Evan Inatome of Elixr Coffee Roasters, who helps operate the Double Knot coffee program. "The tonic gives it just enough sweetness to complement the character of the espresso."

Other spots serving coffee tonics include Rally in Bella Vista. Owner Meredith Waldman said  the key to success is to make sure the espresso is the last component added.

"We pour the shot on top so that right on your nose, you're getting first the coffee aromas and then the tonic," Waldman said. "It makes a huge difference compared to doing it the other way around."

Cafes like Herman's Coffee in Pennsport have been diving into a bit of experimentation by offering tonics spiked with simple syrup options, like a house-made grapefruit-sage version that's now one of its most popular menu items.

"It's become a signature drink of ours," owner Mat Falco said. "I just started playing around with the idea, thinking about what flavors would go with a gin and tonic, and then trying it out with the espresso."

Herman's Coffee offers a variety of C&Ts, including a rotating seasonal version featuring syrups like lavender-lemon, blackberry-thyme, and cranberry-rosemary. For each syrup, Falco likes to blend market-fresh  fruit with some sort of herb, which he'll soon source from the herb garden recently planted outside the cafe.

Herman's also invites customers to take their espresso tonic up a notch through the use of bitters, with options like Aztec chocolate, black walnut, and smoked chili.

"I really love the West Indian orange," Falco said. "It's a bitter from Fee Bros. — the citrusy nature and the bright effervescence of the tonic work really well together."

Providing subtle hints of added flavor, the bitter-dashed elixirs can feel almost as though you're treating yourself to a cocktail — one that's appropriate well before the clock strikes noon.

Fittingly, there's an "Espresso Cocktail" section on Herman's menu. Recent additions include the Pennsport Mule — espresso, tonic, ginger beer, and honey — and the Espressogrino — espresso  over San Pellegrino Limonata. Rally also offers a ginger beer and espresso pairing.

At Function Coffee Labs, which added a coffee tonic to its menu at the start of May, espresso is not part of the recipe. Instead, the shop uses Japanese flash-cooled coffee, a method of preparation in which strong hot coffee is brewed directly over a large chunk of ice.

"We feel that it's less bitter with the tonic and provides a more multidimensional flavor than with a typical cold brew or espresso," co-owner Ross Nickerson said. "The coffee tonic is lighter and brighter, less like the stoutlike elements you get with an espresso tonic."

The Bella Vista shop, which just marked its two-year anniversary, offers two variations. One uses a Colombian bean with notes of brown sugar, honey, and blackberry;  the other uses an Ethiopian bean with flavor notes of grapefruit, jasmine, and blueberry.

Nickerson said he compares the Colombian option to a whiskey and tonic; the Ethiopian version is more like a gin and tonic. Both are lighter in flavor and color than an espresso version.

Green Engine Coffee Co. in Haverford also  uses iced coffee for its tonic, combining cold brew with a house-made syrup. Owner Zach Morris said  the syrup provides for control of the sweetness level, so the drink can be made sweeter for those who prefer it that way.  It still has a bitter complexity beyond coffee, because  the syrup contains quinine, the same ingredient that lends tonic water its distinctive flavor.

"Most coffee shops don't have liquor licenses, so this is a way for us to experiment and have fun," Morris said. "Craft coffee is hitting its stride and is trending toward more adventurous combinations, with coffee serving as an ingredient rather than the only focus."

Want to make one at home, but not lucky enough to have your own personal espresso machine? Experiment with the iced-coffee version of the C&T. Falco suggests starting with a cold-brew coffee concentrate and using tonic to dilute it, adjusting the proportions to taste.

"Try to buy a fruitier coffee, like an Ethiopian or a Honduran bean,"  Falco said. "The fruity notes will go better than a more chocolate-y coffee would with the citrusy flavors of the tonic."

While the type of tonic utilized from coffee shop to coffee shop varies, the majority stand by Fever-Tree, a product free of artificial sweeteners and on the more expensive end of the spectrum.

"It's just really solid tonic," Falco said.

Wondering whether tonic's best mate, gin, can join in the java mix? For those curious about crafting a spiked C&T, Falco confirms that indeed all three can play well together. He shares his recipe for the refreshing spirited beverage below. Grab a bottle of Fever-Tree and some local gin from Manatawny Still Works to try it out yourself.