It's often said that the 2.45 gigahertz used by microwave ovens matches a resonance of the water molecule. Not so: liquid water absorbs energy most strongly at much higher frequencies. Which is lucky, since at those frequencies the energy would all be soaked up at the surface of the food. The frequency was chosen mainly because it falls within radio bands set aside for industrial, scientific and medical use.
Chlorine on its own is fairly gentle. But when it combines with urine, sweat and other grime from swimmers, it forms chloramines, which cause stinging eyes and that classic 'chlorine smell'. A well-kept pool barely smells at all. Shower before you swim and never pee in the water.
Muscle mass slowly shrinks with age, putting independence at risk. But in 1990, a study had nursing home residents aged 86 to 96 train their legs for eight weeks: their strength more than doubled. Muscle responds to training at any age. Getting up from a chair without using your hands is already an exercise.