An open-bite exists when opposing teeth don't meet.

An open-bite can cause eating problems and excessive wear of those teeth which do make contact. An open-bite can be unattractive and can be associated with speech problems.
Open-bites are often caused by abnormal tongue habits and, although the open-bite can be closed with braces, unless the abnormal tongue habits are corrected, the open bite is likely to recur.

Sucking the thumb (or fingers) can produce a localised deformation of the teeth and supporting bone.

Once the sucking habit has been stopped, as illustrated on the right, natural improvement can occur.
If the habit takes years to cease, orthodontics maybe required to reposition the teeth.