Now for more about eyes. If a group has two eyes, then it can never be captured. Why not? Look at this example:
Notice that White can never capture this group. Even if White surrounds the entire outside, Black still has two liberties (one for each eye). If White tries to fill in one of these liberties, the white stone will simply commit suicide. Since white can't fill both eyes at once, the black group will always live! A group that cannot be killed is called a "living group."
It is very important that the group really have two "true" eyes. For example, the group below looks like it has two eyes, but in fact it has one "true" eye and one "false" eye (a false eye is one that can be cut off and killed separate from the rest of the group). Watch how the group can be killed even though it seems to have two eyes:
Now take the quiz below. Do as many problems as you like, then move on to the next page of the tutorial. Make sure that you understand this idea; it is very important, and can be tricky.