Inside of each of us, there are about 10 trillion human cells, and about 100 trillion bacterial cells. By cell count, we are only 10% human.

Given how outnumbered we are, it's surprising that we don't die more often from bacterial disease. You might expect that, of the hundreds of species of bacteria that live inside of us, at least a few would have the habit of getting out of line and growing at our expense.

We can give credit to antibiotics for saving us, but I think that would miss the point. Even before antibiotics, a surprisingly small number of people died from bacteria considering how many of them we host. And if we could invent an antibiotic that would get rid of all bacteria, we wouldn't want to. Our bacteria help us digest our food, store our fats, produce our vitamins, and train our immune systems.

The truth is that we are not alive in spite of the hordes of bacteria that inhabit us. We are alive because of them.

To me, one of the most startling and beautiful properties of our bacteria are their intricate mechanisms for keeping themselves in check. Let's take, for example, bifidobacteria. This species of bacteria lives in our gut and secretes acetic acid, which in turn breaks down the carbohydrates we eat and protects us from certain infections. Remarkably, and luckily for us, the acetic acid produced by our bifidobacteria also keeps them from growing out of control. When the environment gets too acidic, they don't reproduce.

Perhaps I shouldn't be too surprised by this feat of selflessness. Relationships tend to develop a rich texture as they mature, and us and our symbiotic bacteria have been going at this for some time now. I'm reminded of an older couple, where both partners have their quirks, but each knows how far to go, when to pull back, and what to tolerate; where each knows the other so well, and is so dependent on the other, that it's hard to tell where one person stops and where the other begins.

The relationship between us and our tools is newer, like a younger love. It's fiery and exciting, and we're still trying to figure out our boundaries.

Our tools, like most things, have natural limits to their utility. Up to a certain point, e-mail makes us more efficient. After that, the mounds of e-mail in our inbox take time away from our real work. Up to a certain point, time spent on social networks brings us closer to our friends. After that, it takes away from time we spend with them in person.

Our bacteria can offer us some wisdom here. If we want tools that respect their natural limits, we can design limitation into the tools themselves.

If the idea of self-limiting tools seems antithetical to technology and capitalism, let me suggest that we already build them. A search engine is a self-limiting tool. As is an online dating site. When these tools succeed, people leave the site. Video games and TVs, on the other hand, are self-reinforcing. Their use doesn't lead to disuse; their use leads to more use.

The more self-reinforcing a tool is, the more we likely we are to use it at our own expense. On the other hand, the more self-limiting a tool is, the more likely it is to die out.

The key is to find the balance.