Why did rutherford say that bombarding atoms with particles was like “playing with marbles”? what subatomic particle did rutherford discover during this phase of his work?

Ernest Rutherford's discovery was associated to "playing with marble" because it was like hitting a group of marbles and displacing it with the use of a single marble.During his experimentation he bombarded a nitrogen gas with alpha particles and found that occasionally another element, which was oxygen, was produced. He concluded that the alpha particle must have ejected a positively charged particle, which he named PROTON- the subatomic particle he had incidentally discovered.