One day you and your friends were discussing about the matter. He believes that matter is continuous like a block. Initially you have also agreed with him but later one observation changed your perception about matter. Now you believe it is made up of discrete particles. The observation was
Complete dissolution of salt in water
When salt is added to water, it gets completely dissolved in it. So, if matter was like a continuous block, then there must be no space for salt to get dissolved. Hence, matter is composed of many particles.