Simplify the following text for a Grade 7 native English-speaking person.
Original: Hair straightening is a hair styling technique which involves the flattening and straightening of hair in order to give it a smooth , streamlined , and ` sleek ' appearance .
Simplified: There are two different types of straightening : temporary and permanent . Temporary hair straightening keeps the hair straight until it gets wet or damp again , so only lasts for up to a few days .