I'm in Medicine Hat for the Easter weekend. I took along Stephen Hawkings book, "The Universe in a Nutshell", which Anna-Maria gave me for my birthday, to read in the car. This morning I was explaining The Twins Paradox to Tracie, my brother's wife - they are also here visiting. The Twins Paradox goes something like this:

In the theory of relativity each observer has his own measure of time... One of a pair of twins (a) leaves on a space journey during which he travels close to the speed of light, while his brother (b) remains on Earth.
Because of (a)'s motion, time runs more slowly in the spacecraft as seen by the earthbound twin. So on his return the space traveler (a2) will find that his brother (b2) has aged more than himself.
Although it seems against common sense, a number of experiments have implied that in this scenario the traveling twin would indeed be younger.

Tracie responded, "So if you fall in love with someone 30 years younger than you, you can use this to come back and be the same age." I told her I was impressed that she so quickly picked up on the most obviously practical application that this concept enables us to do.