Simplify the following text for a Grade 9 native English-speaking person.
Original: Feynman diagrams are graphs that represent the trajectories of particles in intermediate stages of a scattering process . Unlike a bubble chamber picture , only the sum of all the Feynman diagrams represent any given particle interaction ; particles do not choose a particular diagram each time they interact .
Simplified: The points in Feynman diagrams where the lines meet represent two or more particles that happen to be at the same point in space at the same time .