Chemical substance made in a laboratory. Other chemical substances commonly encountered in pure form are diamond (carbon), gold, table salt (sodium chloride) and refined sugar (sucrose). However, in practice, no substance is entirely pure, and chemical purity is specified according to the intended use of the chemical. Chemical substances exist as solids, liquids, gases, or plasma, and may change between these phases of matter with changes in temperature or pressure. Chemical substances may be combined or converted to others by means of chemical reactions. Forms of energy, such as light and heat, are not matter, and are thus not "substances" in this
Common minnow Hamilton. According to the selfish herd theory, a group forms as individuals try to reduce their domain of danger by approaching others and continuously moving toward the center of the group where the risk of predation is the lowest. As the theory predicts, common minnows increase their shoaling behavior in response to increased predation pressure. Common minnows can detect the predators’ presence and communicate with their shoalmates by a chemical signal that is detected by olfactory nerves. The chemical, named Schreckstoff after a German word meaning "fear substance" by Karl von Frisch who first described it, is contained in specialized