And you also can't say, maybe you might say somehow blood pressure causes more people to spend time in front of a computer. That seems even a little bit sillier, but they're actually the same. Because all you're saying is that there's a correlation, these two variables move together. You can't make a conclusion about causality, that computer time causes blood pressure, or that blood pressure causes some time, high blood pressure causes more computer time. Why can't you make that? Well, there could be what's called a confounding variable. Sometimes called a lurking variable.