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plant would you be?  It’s the question that’s designed to show how innovative you can be, to show how you think:  a zinger.”
    Lee leaned back and nodded.  “Quite the zinger indeed, Nathan.  So, how about it?  How would you stop an alien invasion?”
    Nathan frowned as he thought about Lee’s odd question, and how the entire last few weeks could hinge upon his answer.  He stood up and began to pace slightly.  Nathan never could understand people who could think sitting down.  “Okay, an alien invasion is pretty vague, but defeating any other kind of invasion depends on establishing the parameters of the battlespace.  How are these aliens getting here exactly?”
    “They’re flying here in a giant rocket or rocket-like contraption from a distant star.  They turned around at the halfway point and are decelerating toward Earth, which they will reach in a little over 22 years.”
    “My, my, how specific.”
    Lee had a drink.  “You did say exactly.”
    “Well, from what you’re saying, several methods present themselves, but are we even sure this is an invasion?”
    “No, we aren’t.  Let us say that this hypothetical alien visitor has made no attempt to contact us by signals, or at least there has been no attempt that we have recognized.  Also, the distances are so great that there is no way for us to have yet received a response to our own attempts at communication.  And since we can think of no reason for this unknown alien species to physically come to us other than for invasion, we are proceeding upon a worst-case scenario.”
    Nathan’s pacing was now more rapid, purposeful.  “But they are an alien race, correct?  So ascribing our own reasoning on them is an uncertain proposition, wouldn’t you think?”
    “Of course.  Alien race equals alien reasoning.  Perhaps they are coming here just to say hello, or to plant poppy seeds and welcome us to the inter-galactic love fest.  But why not just contact us with a radio signal?  Radio signals are undoubtedly how they discovered we were even here in this universe, so our common sense would seem to suggest that if their intentions were benign they would have called before stopping by for a visit.
    “Now, that common sense is really just the laws of motion and thermodynamics expressing themselves in our everyday reasoning, laws that the aliens are also bounded by.  If their intentions are benign and they wanted to avoid conflict, it makes sense to make initial contact through signals.  Signals move at the speed of light, and are transmitted at a relatively low power level.  For a species forced to travel below that speed, think of all the time wasted, all the energy wasted in coming here physically.  Signaling us is safer, faster, and cheaper.”
    Nathan stopped and looked at Lee.  “All right, it’s your game.  If you contend that their worries about time and energy expenditure are identical to ours, then their reasoning might be similar to ours as well.  So, the only reason for them to come here is that they need something physical from us, like our resources or our women—Mars always needs women, after all.  And because they didn’t call first or yet, we have to assume their intentions are hostile.”
    “That’s right, but you’re dancing around the question, Mr. Kelley.  How do you stop the potential invasion?”
    Nathan started to walk back and forth again.  “Well, since we’ve established how they’re getting here and confirmed their intentions, the next step is to know the enemy.  We have to conduct reconnaissance at the earliest opportunity.  How far away are these aliens who are taking 22 years to get here?”
    “Let’s say they are about three light-years away now, though they appear four light-years away due to the light speed lag.  They would have slowed from 46% the speed of light to only about a quarter c.”
    Nathan shook his head with a grin.  “You’ve put a lot of thought into this, sir.