Simplify the following text for a Grade 14 native English-speaking person.
Original: Essentially , the theory posits that pressure is due not to static repulsion between molecules , as was Isaac Newton 's conjecture , but due to collisions between molecules moving at different velocities .
Simplified: Essentially , the theory posits that pressure is due not to static repulsion between molecules , as was Isaac Newton 's conjecture , but due to collisions between molecules moving about with a certain velocity .