Simplify the following text for a Grade 8 native English-speaking person.
Original: For this reason , butt plugs tend to be shorter than dildos , and their marked size generally indicates the circumference of the device rather than the length . They must also be very smooth to avoid damaging the rectum or bowel .
Simplified: Butt plugs are similar to dildos , but they are formed so they can not get stuck inside the rectum or anus .