Each eye sees from a slightly different angle. This means that two different pictures are sent to the brain. The brain blends these pictures into one 3D image.

When the eyes don't work together, the brain receives pictures it can't blend together. The brain ignores (suppresses) signals it can't use. In most cases, signals from only one eye are ignored. The signals from the other eye are interpreted as a flat image instead of a 3D image. During suppression, normal vision can't develop in the eye that's being ignored. This is called amblyopia.

Alignment problems (strabismus). One eye looks in a different direction from what it's trying to see. This may happen all the time. Or it may happen some of the time.

Focusing problems. One or both eyes have trouble focusing. The brain receives blurry pictures. Focusing problems include:

• Being nearsighted (distant objects look blurry)

• Being farsighted (nearby objects look blurry)

• Having astigmatism (both nearby and distant objects are distorted). In some cases, focusing problems are worse in one eye than in the other.

Other problems. In rare cases, sight in one or both eyes can be blocked. This can be caused by a problem such as a cloudy lens (cataract). An eye doctor can tell you more about this.