A whiskey sour is a traditional cocktail from the late 19th century.
Whiskey (Traditionally Bourbon).
Sour (typically some kind of lime or lemon juice).
And some kind of botanical bitters (Ango, orange, whatever floats your boat).
And the most important part: The white of one egg.
You put in the whiskey and juice with ice and shake it so it's get diluted and gets cold, then toss the ice and add in the egg white.
This is called a Reverse Dry Shake.The drink is already cold and you build up this nice, rich foam you strain into a double rocks glass and then add your bitters.
And that is how you make a whiskey sour!