Hi, I hope the following explanation could help you out definitely.
J.J Thomson believed that apart from the atom's nucleus, there was also a teeny weeny particle in the atom. By this idea, he proposed that the existence of these particles could be found out. He began an experiment where he bombarded, i.e., shot a foil of gold, so thin, that its thickness was maybe nearly impossible to measure, with alpha particles, which are particles which you should not know in detail about. Moving on, he observed that about 70% of the alpha particles went straight through the gold foil. The rest, came back .This showed that the alpha particles were rebounded by the electrons in the atom.
After this experiment, he said that, "It was as if a bullet was shot at a tissue paper, and it bounced back", since alpha particles are very rapid, when it comes to moving.