Reciprocal, I've heard, is a versatile word,
That I memorized in mathematics,
As when quantities twain yield unity again,
Morphed by multiplied math acrobatics.
Now that may sound obscure and there may be no cure
For defining reciprocal plainly.
Though it may sound perverted, whole numbers inverted
Become their reciprocals, sanely.
If you're paid a poor dime -- reciprocally --
A splendid $10 is what you will see!

Isaac Newton remarked on reciprocal law,
Of the motions of bodies in space,
That for each object's action there'd be a reaction,
A notion that time can't erase.
To change about, correlate, shuffle or commutate,
Play puss in the corner or deal quid pro quo,
Mutatis mutandis, vice versa, or alternate,
Give and take, permutate, swap or change hands,
Lex talionis, tu quoque, or compensate,
These are ways, just like when "castling" at chess, we reciprocate.
You see that wherever the king you move lands,
It's a counterstroke dealt by reciprocal demands!

So make a wish... it might come true.
It's all up to you... it's all up to you!

A mutual attraction is reciprocal love,
An interaction most of us know something of.
On the downside, however, is reciprocal hate --
Suo sibi gladio hunc jugulo, mate!
That's to say "tit for tat," or "catching a Tartar,"
To retaliate, recriminate, retort without barter.
To "exchange a Roland for an Oliver" is hard,
Much like "hoisting (your enemy) with his own petard."
Mete "measure for measure," "turn the tables," or pay off,
To "pay in one's own coin" -- the reciprocal way off.

A contract in law is reciprocal dealing,
Requiring a pact of proportionate stealing.
Conversely, reciprocal plays zero-sum,
As with excited muscles that keep others mum.
But fairness in genomes or physiology
Shows uncommon, amoral reciprocity.
It may be reciprocal vengeance seems lyrical,
But will harvest, I fear, just a crop growing Pyrrhical.
So take my advice: just be just, even-handed --
Repaying unfairness was never commanded!

And make a wish... it might come true.
It's all up to you... it's all up to you!