A simple cooler for the summer, the Highball is just any spirit served in a tall glass with ice and soda.
There are many types of highballs, including the venerable Gin and Tonic, the Moscow Mule, the Cuba Libre (and its degenerate cousin the Rum & Coke), all the various Collinses...the list goes on longer than the summer does.
Made properly, this has only a jigger (an ounce-and-a-half) of liquor and the rest is ice and soda, so it's an easy way to relax on a lazy summer afternoon without getting drunk.
It's not illegal to add a touch of triple sec or other liqueur and a dash of bitters, and then you unlock a whole realm of classic cocktails repurposed as summer coolers!
Fill a tall glass* with ice. Add whiskey, gin, rum, or whatever spirit you like and then top up with seltzer or soda.
I've always wanted to make a Highball with seltzer from an old-style mesh-wrapped soda siphon of the type you see in Three Stooges movies! These are easy to find in antiques shops; unfortunately every last one of them is missing that black Bakelite cartridge holder, without which the device is useless.
* see Notes
A "tall glass" is not simply descriptive. There are standardized glass types, and a tall glass or a highball glass is about 8 ounces, like the one in the photo, with straight sides. If you use your basic beer glass, then adjust the quantities accordingly.