Now let's think more about capturing stones. Look at this stones below:
Thes stones are harder than usual to capture. Why? Because it completely surrounds some empty space. This makes it hard to capture, because for black to capture it, black can't fill up the middle first; the black stones would commit suicide if they tried! Black has to first fill up the outside, then fill up the middle, like this:
These empty spaces inside a group of stones are called "eyes." Eyes can also be formed along the edge of the board. Here are some examples of groups with eyes: