Simplify the following text for a Grade 5 native English-speaking person.
Original: When his friends cut the cord that tied his lawn chair to his Jeep , Walters ' lawn chair rose rapidly to a height of about 15000 f. At first , he did not dare shoot any balloons , fearing that he might unbalance the load and cause himself to spill out .
Simplified: His friends cut the rope that tied his lawn chair to his Jeep and Walters ' lawnchair went up very quickly .