"_These_ mice do. And they'll go on living. For years I've lived in fear that someone would notice and suspect the truth. Just as for years, every time someone has laughed and told me I never seemed to age a day, I've been terrified that he might guess the truth. I'm _not_ aging." Karl looked dazed. "Well, my boy, you've got a bear by the tail. How did you find the elixir or whatever it is?" "You remember the early work with radioactive tracers, a couple of hundred years ago, that proved that all our body cells are in a continuous state of flux? There's a dynamic equilibrium between the disintegration and the resynthesis of the essential factors such as proteins, fats and amino groups, but the cell directs all the incoming material into the right chemical structures, under the influence of some organizing power which resides in the cell. "Foreign influences like viruses may disrupt this order and cause cancer. The cells are continually in a state of change, but always replace their characteristic molecules, and it is only as they grow older that they gradually become 'worn out.' Then the body grows old, becomes less resistant to infection, and eventually succumbs to one disease or another. And you know, of course, that viruses also have this self-duplicating ability. "I reasoned that at birth a man had a definite, finite amount of this essential self-duplicating entity--SDE--in his body cells, a kind of directing factor which reproduces itself, but more slowly than do the body cells. In that case, with the normal multiplication of the cells, the amount of SDE per cell would slowly but surely grow smaller with the years. Eventually the time would come when the percentage would be below the critical level--the cells would be less resistant, would function with less efficiency, and the man would 'grow old.'" Karl nodded soberly. "Reasonable hypothesis." "But one day, by pure chance, I isolated a component which I recognized as being the factor essential to the normal functioning of body cells. It hit me like a toothache. I found that I could synthesize the SDE in the lab, and the only problem then was to get it into a man's cells. If I could do that, keep the SDE level up to that of youth, a man would stop aging! Since viruses penetrate our cells when they infect us, it was no trick at all to effect a chemical coupling of the SDE to the virus. I used Martian Blue, since it was handy, and its effects are usually brief. "Presto! Old age is held at bay for another twenty or thirty years--I really don't know how long. These mice were my first experiment, and as you see, they're still alive. Next, I tried it on myself." David put the mice back in their cage, locked it, and returned to the lab.