Which tool is commonly used for achieving curls in straight hair?

The tool commonly used for achieving curls in straight hair is the curling iron. This device is specifically designed to create curls by using heat to shape the hair around a heated barrel. The curling iron allows for different barrel sizes, which gives the stylist the flexibility to create various types of curls, from tight ringlets to loose waves. The mechanism works by wrapping sections of hair around the barrel and holding it in place, allowing the heat to set the curl.

In contrast, a blow dryer is primarily used to dry hair, and while it can be used in conjunction with other styling tools or techniques to create volume or bends, it does not inherently curl the hair on its own. A straightener is designed to smooth and straighten hair, which is the opposite of curling. Lastly, a hairbrush is used mainly for detangling or styling but does not have the capability to curl hair without the aid of heat or other curling techniques. Therefore, the curling iron is clearly the best choice for this purpose.