B. From this prohibition (Negative Mitzvah #187, to be exact), The Rabbis derived the Halachah (Jewish law) that any meat product may not be eaten with any dairy product. (From the same passage is derived Negative Mitzvah #186, which separately prohibits cooking, baking or otherwise mixing meat and dairy products together.)

C. So what's so terrible about eating meat and milk together? What's G-d's problem with it? We don't begin to know the whole of it, and we are glad to do it as a Divine decree!²  Nonetheless, some insight into this decree has been shed and here is a Kabbalistic perspective: milk symbolizes life and meat symbolizes death, and combining the two creates a spiritual clash in the celestial realms both of your soul and in the worlds beyond us.

How do I avoid eating meat and milk together?

1. Segregation in the Service

Not eating meat and milk together begins with avoiding situations that might bring the two together in the first place. So start with a Kosher kitchen--such a place contains two separate countertop workspaces, and two separate closet spaces each containing a complete set of dishes, cutlery, pots and pans and utensils. According to physics (and you can ask Julia Child about this), metals (and certainly woods) can "absorb" and become impregnated with the "flavors" or residues of the foods prepared with or in them. Thus, your favorite meat stew pot may not be used to make real hot cocoa (which calls for hot milk), because the cocoa milk would have meaty overtones. Additionally, you'll need separate ovens, microwaves and stovetop burners for meat and dairy products, respectively. (Many kosher kitchens simply have two separate ovens.)

2. Take Your Time

After enjoying a hearty pastrami sandwich or any other meat product, halachah rules that you must wait a minimum of six hours before falling upon your beloved mozzarella. Here, human biology comes into play: since it takes the stomach about six hours to fully digest the proteins known as meats, if any dairy product enters the stomach during that time, the stomach will process and churn both foods together. Jewish law considers this to be eating meat and milk together, so do hold off on the pizza if you've just wolfed down a steak.

The situation generally works the same in reverse--you must wait some time after dairy stuff before you eat meat. Some dairy products wend their way through the digestive system faster, so one would only have to wait a few minutes, or an hour, after eating them before moving on to meat. Some cheeses, though, like aged or fine Swiss cheeses, do take six hours to fully break down and absorb into the body, so you may have to wait that long for your pastrami sandwich if you just enjoyed some fine Wisconsin fare.

3. Ask a Rabbi

There are tons of culinary situations that arise in the frenzied, messy madness we know as home cooking--with a rabbinical response for each. What if you put a milk-only fork in a bubbling pot full of hot dogs? How about if you baked some Eggplant Parmesan in your meat oven? That's just a rabbi is for, and it should come as no surprise that two of the four huge sections of halachah whose knowledge is de regeur for rabbi wannabes deal exclusively with meat and milk issues. So if you've got a question about your kosher kitchen, either AskMoses or contact your local Chabad rabbi via chabad.org.