Apple designs iPhone touchscreens to stand up to falls and other abuse by using an super strong type of glass called aluminosilicate backed by a layer of aluminum. This greatly decreases the chance that the screen will crack, but it doesn't eliminate the risk completely. Once the screen does crack, the possibility of the crack spreading can be high. However, a fast-acting liquid adhesive such as super glue or nail glue can fill in the fracture and hold it together until you have it replaced. This stops it from getting longer or branching off to create more cracks. Although the area around the break may experience a decrease in sensitivity or accuracy, you will be able to use the phone normally without making the crack spread.
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Place a small amount of glue onto the end of a toothpick. It should be enough to coat the point without dripping off.
Spread the glue into the crack with the toothpick. Keep the application thin so it can dry smoothly and evenly.
Moisten the tip of a cotton swab with the nail polish remover and wipe off any excess glue around the crack. Be careful not to wipe away the glue in the crack itself.
Wait 10 to 15 minutes for the glue to dry completely. You can then resume normal use of the touchscreen.