It is use by Eye Care Professionals To check the eyesight of a person.
If u would have visited a eye care centre u would have noticed a chart like this--
If a person can see all the letters from a distance his eyesight is Perfect.If that person misses a letter or a line from the grid eye treatment or spectacles are given to the person for improving their eyesight
Okay, this is an interesting one but mainly no, most activity's will so you don't push yourself but if you really wanted to you could go for however long you want as long as you are not doing harm to your body, depending if you are on a fitness program there will be limits.
I don't think it's supposed to keep it numb but if I was to guest at least a few minutes I don't think it goes past 3
The Three Ways Are
1. Keep your surroundings clean.
2. Wash before and after eating.
3. Visit doctors regularly.
Hope this helps.
For exercise to be efficient, you must engage in a meaningful exercise
at least three to five times a week. Exercising too little won't engage
your muscles and body like it should. Conversely, too much exercise on
your body won't allow it any time to heal itself from strain and
exertion.