Which tool is primarily used to create curls in the hair using heat?

The tool that is primarily used to create curls in the hair using heat is the curling iron. This device is specifically designed to generate heat and shape sections of hair into curls. Curling irons come in various barrel sizes, allowing for different curl types, from tight ringlets to loose waves. The heating element of the curling iron allows the hair to hold the curl in place once it cools, making it effective for styling.

In contrast, while a blow dryer can be used to style hair and create volume, it does not inherently shape hair into curls. A flat iron, although capable of creating waves, is mainly designed for straightening hair and is not typically associated with curling as its primary function. A round brush is often used in conjunction with a blow dryer to add volume and curl when styling, but it does not provide the same level of defined curls that a curling iron does. Thus, the curling iron stands out as the definitive tool for creating curls with heat.