Which ingredient is common to both a Tom Collins and a Sloe Gin Fizz?

The common ingredient between a Tom Collins and a Sloe Gin Fizz is soda water. Both cocktails incorporate soda water to add fizziness and balance the flavors of the other ingredients.

In a Tom Collins, the drink is typically made with gin, lemon juice, simple syrup, and soda water, giving it a refreshing, bubbly character. Similarly, a Sloe Gin Fizz features sloe gin (a gin flavored with sloe berries), lemon juice, and soda water, creating a light and effervescent drink.

While gin is a primary ingredient in a Tom Collins, it is not present in a Sloe Gin Fizz, making it an incorrect choice for the question. Sour mix, usually comprised of lemon or lime juice and simple syrup, is also not used in both cocktails; it's typical in many drinks but not specifically in the Sloe Gin Fizz. Lastly, cherries are not a key component in either drink, even though they might sometimes be used as a garnish in various cocktails. The focus here is on soda water as the unifying ingredient that provides the sparkling quality that both cocktails are known for.