In addition to the flashlight (corneal reflectance examination), the doctor’s magic tool for diagnosing strabismus is the masking examination. This test determines whether the child has intermittent strabismus or occult strabismus. Occult strabismus is an underlying eye position deviation. Because our eyes have a fusion function, which means that when both eyes look at the same time, the brain center can combine the object images that fall on the retina of both eyes into one complete object image. Under the control of the fusion function, when we look at both eyes at the same time, the eye muscles will control the eye in a positive position without deviation, but if the fusion function is disturbed such as covering one eye, it will show the eye position deviation. Cryptorchidism is very common and does not require treatment when there are no symptoms. Intermittent strabismus is a dominant strabismus in which the fusion function of the brain is sometimes good and sometimes bad, with corresponding manifestations of sometimes orthogonal and sometimes deviated eye position. Occult strabismus and intermittent strabismus are difficult to detect when both eyes are seeing at the same time, so a cover-up exam is required. As the name implies, masking examinations prevent the child’s ability to see with both eyes at the same time by covering one eye, which disrupts the fusion of the eyes, and then observing the eye movements to determine whether the child has intermittent or occult strabismus. The masking test consists of the “masking and de-masking test” and the “alternating masking test”. In the masking-uncovering test, the child is asked to look at an object at a distance of 33 cm or 6 meters, cover the right eye for 2 to 3 seconds, and observe the performance of the left eye; if it does not move, it is not strabismus; then cover the left eye in the same way, and observe the right eye to determine whether there is strabismus. After the masking de-masking test except intermittent strabismus, the alternating masking test is performed again, at which time the left and right eyes are alternately masked and repeatedly alternated several times to observe whether the eyes move after the masking is removed. In a normal child, the eyes do not move during the alternate masking process; if the covered eyes move when the mask is removed, the child has occult strabismus. In addition, some children have what appears to be an “opposite eye” (internal strabismus), but it is actually an inner canthus, which means that there is more skin near the corner of the eye on the nasal side, covering part of the “white eye”, the sclera on the nasal side, and giving the illusion of an internal strabismus. This condition is most often seen in Asian populations. Strabismus can be ruled out by a masking examination. Parents can also pinch the skin on the bridge of the nose with their hands to see if the child has strabismus. The masking test is simple and easy to perform, and parents can also try the masking method at home to make a preliminary determination if they suspect that their child has strabismus. However, the masking test requires a high degree of cooperation from the child and often needs to be repeated several times, the test results may not be accurate, parents should not jump to conclusions on their own, and need to go to the hospital as soon as possible for a clear diagnosis.