The easy answer is, if you have to ask, it's probably lust.

The harder answer is a bit more complicated.

You may or may not know this, but when you first fall in love with someone, your body starts producing its own natural amphetamine - phenylethalimine (PEA). Plus, your brain is also making and being flooded with dopamine, norepinephrine, and serotonin. Also referred to as "morphine of the brain", this is good stuff! These natural "love drugs" are stimulants, painkillers, and, yes, aphrodisiacs, because they increase the libido (that internal brain thing that makes you want to have a lot of sex). This is why new couples tend to spend a lot of time in bed!

So at first, thanks to your own brain chemisty, it's love and lust together.

The trick here, if you're interested in growing old with someone, is to not get addicted to the first rush of all these cool natural highs. Because they, like all drug-induced highs, don't last.

The question then becomes, "Am I really in love with this person, or just his or her body?"

I'm old enough to remember a syndicated comic strip back in the 1980's and early 1990's called "Bloom County." At one point Steve, a former frat boy and the self-proclaimed "studmuffin" of the strip was dating a stacked bimbo named Quiche Lorraine. Steve was in an accident (never mind the details, they're too weird) and put in a body cast for several months. Then ensuing dialogue went like this:

Quiche: A confession Steve...I no longer love you. The truth is that I was only interested in your body.

Steve: My body?

Quiche: Your body.

Steve: Just my body?

Quiche: Yes. And now it's broken.

Steve: And my mind?!

Quiche: ...is cabbage.

And she left. I guess the "brain morphine" wasn't enough to see them through.

So here are some questions to ask yourself to help clarify where you are on the love/lust spectrum.

1. Do you honestly see yourself growing old with this person? If you answered yes, it's love. I don't know about you, but love for me incorporates the idea of sitting on a porch someday with my husband Alex, arguing over whose false teeth are whose. We laugh about this a lot. Someday it may not be funny!

2. If you could never have sex with this person again, how long would you stay? A week? A month? The rest of your life? How long would you stay if your relationship vows to this person prevented you from having sex with anyone else? Think about how long Christopher Reeve was completely paralyzed before he died. Obviously, sex as they knew it was over for him and his wife, but she stayed by his side - literally and figuratively - until the day he died. Another "yes" answer means it's love!

3. If this person has a physical or emotional problem, what is your response? Say the girl you've been dating is in a car accident. No injuries, but she's bruised enough that sex is out of the question for a few days, maybe a week. Is your first instinct to comfort her and be there to cuddle her until she stops waking up from nightmares of the wreck? This would be the option you'd choose if you're in love. Or is your first instinct to call another girl you know and arrange for a hook-up later that same night? After all, why should you be deprived just because your girl can't perform? Do I even have to tell you what choosing this option means?

The "love drugs" your body produces naturally whenever you fall for someone may not last, but if your relationship gets to the point where it doesn't need them, well, then you just might have something.

And that something just might be love.