ANSWER: Time is the most used noun in English (probably because we spend too much time agonizing about having too little time), but time is still a puzzle. Until Einstein burst our bubble, we thought time ticked away predictably. One consequence of his theory of relativity is time dilation – slowing of time at high speeds.

Time dilation is not just a crazy result of the complex mathematics of relativity. Atomic clocks on the space shuttle run measurably more slowly than clocks on Earth. The difference is tiny, but time dilation becomes significant near light speed, giving rise to the twin paradox. If an astronaut took a long voyage at near the speed of light, he would return to find he was much younger than his earthbound twin.

Length contraction is another consequence of relativity. If the Earth twin watched his rocket-man brother eat dinner on his voyage, length contraction would make the food appear smaller, but time dilation would make the meal last longer according to Earth clocks.

In other words, space and time are intimately connected in four-dimensional spacetime – three dimensions of space and one dimension of time. Some researchers have proposed that there are hidden dimensions of space and perhaps even additional dimensions of time.

Gravity is also full of mysteries. Newton's law of gravitation – particles attract each other with a force proportional to the product of their masses and inversely proportional to the square of the distance between them – works for everyday situations. However, it was revealed to be incomplete when it failed to account for the irregularity in Mercury's orbit around the sun.

Mercury's variable orbit and the bending of light around massive objects are explained by Einstein's interpretation of gravity. It states that the presence of mass distorts the fabric of spacetime, and freely moving objects follow this curved path.

Sudden movement of very large masses should cause ripples in spacetime. These gravitational waves have not yet been detected directly. More troubling is that the universe's expansion seems to be speeding up, which implies there is an unknown anti-gravity force or a flaw in our understanding of gravity.

Things break down on the smallest scale, too. Gravity refuses to fit into the standard model – the theory that explains how the other fundamental forces hold matter together. So gravity is preventing physicists from having much spare time.

Sherry Seethaler is a UCSD science writer and educator. Send scientific questions to her at Quest, The San Diego Union-Tribune, P.O. Box 120191, San Diego, CA 92112-0191. Or e-mail sseethaler@ucsd.edu. Please include your name, city of residence and phone number.