Albert Einstein, one of the smartest people whoever lived, thought about this question for 20 years before he even began to understand it. And today astrophysicists and other scientists are still probing the mysteries of time.

Although in the every day world time and space seem unrelated, Einstein showed that in reality they are deeply related. In the vicinity of massive objects like black holes in space, the fabric of space time becomes disrupted and it may even be possible to travel back in time or to go to different regions of space. These are the ideas that are at the frontier of knowledge.

In more practical terms, we tell time by measuring the number of times an electron in the element cesium vibrates back and forth in the time period the earth goes around the sun one time (a year).

Time is a mysterious entity that perhaps someday we will have a better notion of.

Keep on thinking because I don't think anyone really knows the answer! But I'll try to explain the little bit I understand with an example:

Suppose my friend Jon tossed an egg across the room and I catch it. Now suppose we used a video camera to film the egg fly out of Jon's hand, across the room, and into Mark's hand. Play the video frame-by-frame. We stop at a frame where the egg is halfway across the room. That camera has recorded the position-in-space of the egg at that position-in-time. When I say "position-in-space," I don't mean where astronauts go. I mean, "the egg is 3.1 meters off the ground, and 4.7 meters in front of Jon's hand." When I say "position-in-time", I mean,"0.2 seconds after the egg has left Jon's hand." What units is "position-in-space" measured in? What units is "position-in-time" measured in?

Now suppose you didn't know what happened, and you were looking at the video for the very first time. Now I have the remote control for the VCR, so you don't know that I have pushed "reverse." You see the egg flying from Mark to Jon, instead of from Jon to Mark.

Which possibility can you correctly conclude?

In this case, the correct answer is C.

Does this mean that the only difference between "position-in-time" and "position-in-space" are the units in which we measure those quantities?,

Now suppose that I was a klutz, and instead of catching the egg, it broke in my hand. Again, you see the tape played in reverse, but you don't know that I pushed reverse.

Which possibility can you correctly conclude?

Of, course, B is correct. An smashed egg cannot spontaneously become whole again.

What is going on here? It seems that Time has an ARROW. It goes FORWARD. But why does the breaking egg only make sense breaking apart instead of coming together, while the flying egg makes sense going either left or right?

Physicists are trying to understand Time by asking these and similar questions. They take an event (sort of like our egg toss, except they use microscopic particles) and ask, Which possibility can we conclude?

Remember, keep thinking about this because nobody really understands it, especially me!

A famous physicist once said that "time is what keeps everything from happening at once". This is a funny answer, but in a sense it is true: time is a distance between events, just as space is a distance between places. We measure the "time distance" in seconds, just as we measure the "space distance" in inches, etc. One important difference between time and space, though, is that the two directions of are very different from each other: we are continually moving into the future, and cannot move into the past. In space the situation is quite different: we can move in any direction equally easily. It is still not understood by scientists why the two directions of time are so different from each other.

A tough question indeed. The best answer is time is what you measure with a "clock". This may seem like a sarcastic or "wise guy" sort of an answer but there is no better answer.

There a few other things like time that can be only defined "operationally". That is you can only define them by how they are measured. Length is another. Length is something you measure with a meter stick or yard stick or ruler. You can not do better.

A third thing is mass. Mass is something you measure with a"scale".

Having said all that you are safe to define other physical quantities in terms of these basic quantities, time, length and mass.

For example momentum is a mass moving a certain length in a certain time. It can be defined in terms of these concepts. It does not need to be defined "operationally"

I hope you get a feel for these "concepts".