Albert Einstein theorized that time flowed at different rates. According to him, the faster one traveled, the slower time flowed. Einstein called this phenomenon time dilation, and invented the "Twin Paradox" to explain its effects. Click the above graphic or here to see an interactive example (requires Shockwave Player(1)) of the "Twin Paradox."
INSTRUCTIONS:
Click people (green) to set age
Click ship to set velocity
Click on a star to launch ship
After the ship returns, note the astronaut's age compared to the age of the person who stayed on Earth. Adjust the options and relaunch to see how things change.
Credit:?! Productions

1. http://www.macromedia.com/software/shockwaveplayer/