Of course living matter is not injured by the mere presence of a gamma ray. There is a small probability that the gamma ray could go right through the body without a single encounter. If so, there would be no biological effect. An effect is produced only when the gamma ray interacts with the matter. There are three most important ways in which such an interaction may occur. One way is simple _absorption_ of the gamma ray by one of the atomic electrons. The gamma ray disappears in this process, and the electron acquires all of its energy. A tiny bit of this energy is used for the electron to break its bond with the atom. The remainder goes into kinetic motion of the electron. The electron is now on the loose and can cause biological damage by exciting and ionizing other atomic electrons. In fact it is now the same thing which we used to call a beta ray. A second way in which the gamma ray may interact with matter is by _scattering_. In this case the gamma ray does not disappear but merely loses a part of its energy to the atomic electron. Again the electron is free to cause biological damage, while the gamma ray goes on to its next encounter. The third way requires that the gamma ray be near a nucleus and have an energy greater than a million electron-volts. (Ordinary X-rays such as are used in medical practice are not energetic enough for this process to occur.) Under these conditions the gamma ray may disappear, with the simultaneous appearance of an electron and a positron. This is an example of the creation of matter out of pure energy. In accordance with the formula E = mc², a part of the gamma-ray energy is consumed in producing particles with definite masses. This amounts to about one million electron-volts. The remainder of the gamma-ray energy goes into kinetic motion of the two particles. Again biological damage results from the subsequent ionization due to the charged particles. After the positron has expended its kinetic energy in the ionization process, it will join with an electron in a disappearing act. The energy reappears in the form of two or three gamma rays (each having less energy than the original gamma ray). In no case is the gamma ray directly responsible for any biological damage. The damage is always made by electrons (or positrons) to which the gamma ray has transferred some or all of its energy. But this only makes gamma rays the more dangerous. They can first penetrate to the sensitive tissues of the body, and then cause ionization. We have already mentioned that X-rays are the same as gamma rays. The latter are produced by an excited nucleus, the former in the collision of an electron (or a beta ray) with a nucleus. The man-made X-rays are obtained by first accelerating a stream of electrons and then letting them impinge on a target containing highly charged nuclei.