Predicting earthquakes using observable phenomena like phases of the moon, cloud formations, weather conditions or animal behaviour is a controversial and, some would say, impossible task. There is little or no verifiable evidence to link any of these phenomena to predictions of actual seismic activity. Early detection, as a means of advance warning, is far more reliable than future prediction and can be done by measuring primary waves (referred to as P-Waves). Prediction is much flakier science than detection. These non-destructive waves travel faster through the earth's crust than the harmful and destructive secondary waves and so can provide a valuable, albeit short, advance warning. This effect of different speed waves can be likened to the relationship between light and sound; we see lightning before we hear the thunder. Some people believe that animals can detect P-Waves and that this is what causes them to start behaving strangely. Other possibilities of animal’s behaviour shortly before an earthquake are that they are reacting to the low-frequency electromagnetic signals caused by the fracturing of crystalline rock during the early stages of an earthquake.