Anyone who has raised children or ever was a child (and let's face it, that's all of us), knows that the world's Most Frequently Asked Question is "Why?" Unfortunately, it's also the most difficult question to answer because it's bottomless - it can keep right on going forever because no matter how you answer the first "why" there's always a follow-up just waiting to be voiced.
So here is a collection of the all-time best follow-up-stopping answers to the perennial question. There is no particular order, since those who answer the question are the ones who must ultimately judge the adequacy and value of their own answers. Everything depends on how deep one cares go into sideways diversions, or whether one feels that an abrupt cut-off to the line of questioning is required.
1. Because I Said So.
This perennial favorite of parents everywhere can be either the beginning or the ending to a line of "why" questions. It is usually a resort to something dimly remembered from one's own childhood and teenage existential angst. Generally preferred when a questioner is either too young or too headstrong to understand all the many good reasons a parent won't let them play in the street or borrow the car to attend the Senior Drunk. It's the ultimate in 'Appeal to Authority,' and brooks no sustained follow-up. Obviously, this is an appeal to authority that works much better when dealing with a willful child than when offering testimony before a Congressional committee, so consider the audience and use this one with a healthy sense of relative power distributions.
2. Because God Says So.
This answer to the "why" question hinges on one's preferred metaphysical belief system as well as the accepted belief system of the questioner. Depending on the context, this answer can be used to good effect in a number of different applications dealing with moral or ethical issues, or even some deeper existential "why's." The person who uses this appeal to ultimate authority should probably be prepared to back it up with actual scriptural reference if dealing with anyone other than immediate family. It may work very well when chiding religious fundamentalists who turn out to be philanderers, liars, crooks or homosexuals, even in the political arena. But it's probably not a good answer when dealing with people who don't believe in God.
3. Because Adam and Eve Messed Up.
On a deeper level of questioning, a reverse-psychology appeal to the Judeo-Christian archetypal ancestors could work better than an appeal to the absolute authority of deity itself. In the oral traditions of the early Semitic tribes comes the sad tale of the first created humans, Adam and his wife Eve. It signifies an attempt to contextualize the difficult situation in which humanity finds itself while existing in the physical world. A "fall" of perfect creation into sin is described as the result of their choice to eat of the fruit of the Tree of Knowledge (of Good and Evil) in defiance of God's prohibition. This appeal to the weakness (non-authority) of archetypal ancestors is often useful within those religious traditions to help people understand - and resist - their desire to do the wrong thing.
4. Why Not?
This is an answer with lots of energetic 'oomph' behind it, most useful if you happen to be the one doing something your questioner finds completely unfathomable. Best when not used in any more esoteric moral or ethical context, yet positively infectious when used to justify unpredictable behaviors that happen to occur on a spur of the moment basis and cause no real harm. Recall the scene of George and Mary at the high school dance in the movie "It's a Wonderful Life," when after they've fallen into the under-floor swimming pool the rest of the party-goers impulsively jump right into the water too. The old biddies are horrified even as their own husbands delightedly leap into the pool wearing their tuxedos. Why not?
As people get older, their "why" questions do tend to become ever more existential. The "why" seeks reason, and to qualify as a proper answer, the answer must have meaning behind it. In Douglas Adam's popular book "The Hitchhiker's Guide to the Galaxy" the number 42 is given as the ultimate answer to the meaning of life, the universe, and everything. 42 is also the atomic number of the element molybdenum, the angle in degrees of a rainbow, the Messier designation of the Orion Nebula, the number of generations in St. Matthew's genealogy of Jesus and the number of months the Beast holds dominion in the End Times. There were 42 principles of Ma'at in ancient Egypt leading to the personification of 42 gods. And in the Babylonian Talmud and Kabalistic tradition, there are 42 letters in the secret name of God.
6. Soul Evolution.
In keeping with the deeper meaning of the "why" question - which require quite a bit more explanation than the old standby "because I said so," many human spiritual traditions postulate that existence in the physical world is a stage in the evolution of the soul toward some better future condition. Depending on the tradition's belief system, souls may incarnate along the so-called "Great Chain of Being" from microbes through insects to animals, and finally to people. People may then incarnate across the spectrum of conditions until finally reaching "enlightenment," at which point they either go on to the next, non-physical plane of existence or become 'one with' the universe or "oversoul" of being. This answer doesn't justify much in real life, but it may help people cope with unreasonable or unjust experiences.
7. Quantum Fluctuation.
"Why" anything exists rather than nothing is probably the most existential of 'why' questions, but it is one that science has attempted to answer just as philosophy and metaphysics traditionally have. In the past century there have been some significant successes in the realm of physical theory toward this end, not the least of which has been quantum mechanics. According to some theorists the universe exists due to a random fluctuation in the sub-Planck level "quantum vacuum." While not very satisfying as an answer to "why," it can be used to excuse almost any existential condition with a cosmic shrug of the proverbial shoulders. "It just is," and that's equivalent to "because I said so" when dealing with the emotional equivalent 2-year olds in adult bodies.
8. White Hole.
Ever since scientists have been concerned with explaining that most existential of "why" questions without recourse to deity(ies), they've also tried to trivialize the apparently fine-tuned parameters of the universe we live in - those which allow for matter and life to exist - by postulating an infinite number of other universes that exist, but that we can't detect or know anything about. In a few of those type of "multiverse" models our universe began at the opposite end of a black hole (cosmic singularity) in some other universe - a so-called "White Hole" out of which all things emanate as if from a Big Bang after they've been sucked out of existence in that 'other' universe. We don't observe white holes in our universe, though according to the same solutions of General Relativity that give us black holes, we should. Perhaps if our whole sheebang is a white hole, we don't need any others!
9. Colliding Branes.
A new and improved take on the quantum fluctuation answer to "why" is the colliding brane scenario. This one is fundamentally as random as the quantum fluctuation answer, but so additionally complex that it will impress almost anybody - or at least make them dizzy enough to drop the line of questioning quickly. Even better, it has its own cool title that can draw oohs and aahs - the "Ekpyrotic Universe." It doesn't entirely replace the Big Bang (from quantum fluctuation or white hole or godly command or whatever) but has more dimensions, and this makes it cooler. Visualize sheets of paper floating through the fifth dimension, if you can visualize what that looks like. When these sheets collide, it causes Big Bangs - yes, there are more than a few in this theory. Out of which expands new universal branes like (or unlike) our 4-dimensional space-time.
10. Anthropic Multiverse.
This answer may be the best non-religious answer of all, given that it ultimately boils down to "because you are here asking the question." That's always a sure-fire query-stopper! In this one there are still an infinite number of universes out there that may or may not look anything like ours (and which we'll never know for sure exist or anything about them), but we happen to live in the one we live in because the laws of physics in THIS universe allow us to exist. If any of those 'fine-tuned' constants were any different from what they are, we wouldn't be here to ask bottomless existential questions, so there you go. This is another version of the "it just is" answer, but again, so impressively cutting-edge that most people who aren't physical theorists won't bother asking the next "why" in the series.