i mean if you put your hand up to the sun you can feel it, you detect it, your eyes detect it, well you have detectors which detect them and, for example, if i want to detect something like an electron well then i can make a counter which is sensitive to charged particles like electrons, and i can allow these electrons to hit this counter and it will produce erm an identifiable electrical pulse and i can look at that and i can say this is an electron, or i can look at other particles, say, for things like helium nuclei which are called alpha particles, and i can make counters which will detect these and i can put a little piece of paper in front and i can stop off the alpha particles.