This SpaceTime Lab lets you explore how a planet and a black hole affect the space and time around them.

Rules of the game

1. The green circles mark off equal distances in space.  
2. You can measure space by sliding the ruler towards or away from the central object.
3. You can measure time by sliding the blinking clock towards or away from the central object.  
4. You can add or take away mass from the central object by clicking on the arrows. This makes the object heavier or lighter. (The size of the object remains the same in this game.)

Experiments: Making a black hole
Add some mass to the central object by clicking the arrows. What happens to the circles?
The more mass in a given volume, the more the space and time around it is distorted.

Keep adding mass until the slider says "black hole." 
If enough mass is crammed into a given volume of space, you create a black hole.

Slide the blinking clock closer to the black hole. What happens to the clock?
To a distant observer (that's you) a clock appears to run slower as it gets closer to a black hole. At the edge of the black hole, time appears (to us) to stop completely!

Slide the ruler closer to the object. What happens to the ruler?
To a distant observer (that's you) a ruler appears to shrink as it gets closer black hole. Near the edge of the black hole, the ruler appears to shrink to nothingness!

Decrease the mass of the object using the left arrow. What happens to the ruler and clock now?

What's going on?
You might think that an inch is an inch and a second is a second no matter where you are in the universe. But Einstein discovered that the scale of space and time are distorted by any massive object. For example, the closer you are to a mass, the more time slows down. And the more massive the object, the more time slows down. On Earth, the effect is too slight to be noticed. Near a black hole, the effect is extreme.