Islam stresses the importance of personal hygiene and sanitation. Among the prophet's teachings about personal cleanliness is the use of siwak to brush and clean one's teeth.
 

Siwak is a long stick, usually cut into short pieces, from a tree known by the name" Alaraak". The prophet always advised his companion to brush their mouths using" Siwak", the prophet says:

Abu Hurayra reported that the Messenger of Allah, may Allah bless him and grant him peace, said, "If it was not for the fact that I might have been overburdening my community - or the people - I would have ordered them to use the siwak for every prayer." [Agreed upon] Riyad as-Salihin, imam Nawawi
 

The prophet used to use Siwak before he slept, when he got up, before starting his prayer and even  he used it during fasting. The importance that the prophet attached to the use of Siwak must imply a scientific miracle and that what an American team has proved
 

The scientific facet in the prophet's words

The frequent use of Swiak by the prophet and urging his followers to use it to such a degree implies a secret behind the use of Swiak; many tests were conducted on the effect of swiak  and all prove that it excels the use of toothpaste in killing micro-organism and stopping its effect.
 

Mouth, owing to food left-over, is a good fertile for microbes and Bacteria to grow and multiply, these cause teeth to be decayed, gum to be cut and bad stench. Unlike toothpaste, the effect of Siwak  on teeth lasts to eight hours while the maximum effect of toothpaste is two hours, it has been proven that Siwak contains nineteen important natural materials that can be classified as follows:
 

Antibacterial acidic inhibitors that fight decay and diarrhea. They are natural disinfectants and can be used to stop bleeding. They disinfect the gums and teeth and close any microscopic cuts that may have existed in the gums. On first usage, the siwak will taste harsh, and maybe even burn, because of a mustard-like substance found in it, but this is the ingredient that fights decay in the mouth and kills germs.

Minerals such as sodium chloride, potassium, sodium bicarbonate and calcium oxides. These clean the teeth. For instance, the American Dental Association considers sodium bicarbonate to be a preferred ingredient in toothpastes.
 

Natural scented oils that taste and smell nice, give the mouth a nice smell. They make up about 1% of the siwak. Enzymes that prevent the buildup of plaque that causes gum disease. Plaque is also the no. 1 cause of premature loss of teeth.
 

Anti-decay and anti-germ ingredients that act as a penicillin of sorts, decreasing the amount of bacteria in the mouth, which means cleaner teeth and cleaner air when breathing through the mouth.
 

http://www.islamonline.net/iol-english/dowalia/techng-22-03-2000/techng2.asp

The question we would like to ask here, how could Mohammed  knew all of these facts unless he was a prophet sent by Allah(God)?