This post will be about something you do twice a day (and if you don't, please do it for the sake of the people around you). This is about brushing your teeth. Essentially you are scrubbing your teeth with an abrasive.
Now, why do we need to do this? Well, we humans have a great appreciation for sweetness and the taste of sugar. The industrial revolution made production very cheap so it started to be added into everything. In fact sugar is presumed to be in everything we eat that some items without sugar have to be deliberately marked as 'sugar-free'. Sugar is a carbohydrate - you eat it and it gives you a lot of energy. When you eat or drink it it sticks to your teeth. Bacteria in your mouth feeds off this sugar and sticks to your teeth forming plaque. The byproduct of this bacteria feeding is acid and over time this breaks down your teeth causing cavities...
We brush our teeth to remove this plaque from our teeth - stopping it from doing any damage. Turns out that this act of brushing can actually do damage to the teeth. Toothpaste+toothbrush is an abrasive that scrubs plaque off teeth. If done directly after eating or drinking (especially acidic foods) it can actually cause loss of enamel which is the stuff teeth are made of. Consuming high acidic foods causes the enamel to soften and in cases like these it is better to leave the teeth as it is instead of brushing it.
Flossing is the best way to go as it removes bacteria without being harsh on the teeth. It is still recommended to brush your teeth twice a day, just do it away from meal times and use a soft brush.
By the way, funny thing happens when you drink orange juice right after you brush your teeth.