Neutrinos are very difficult to detect. They are very unlikely to collide (interact) with other particles as they travel through space or through matter. Electrons have an electric charge, but neutrinos do not. This means that neutrinos are unaffected by the electromagnetic force. The first detectors built to find neutrinos observed only 10 or 15 each year. Recent detectors use tanks that contain about a thousand tons of water or other liquid, which lets them detect about 10 to 100 neutrinos per day.