Simplify the following text for a Grade 8 native English-speaking person.
Original: Straightening irons , straighteners , or flat irons , not to be confused with clothing irons , work by breaking down the hair 's hydrogen bonds found in the hair 's cortex , which cause hair to bend and become curly .
Simplified: Hair straighteners work by breaking the hair 's hydrogen bonds , which are what make the hair curly .