Simplify the following text for a Grade 8 native English-speaking person.
Original: It is not possible to state that a certain gas causes an exact percentage of the greenhouse effect . This is because some of the gases absorb and emit radiation at the same frequencies as others , so that the total greenhouse effect is not simply the sum of the influence of each gas .
Simplified: Most greenhouse gases are natural - water vapor is the most common , and causes most of the greenhouse effect on Earth .