What connects an island to the mainland?

An island is NOT connected to the mainland, by the very fact that it is an 'island'.

However, there are 'peninsulars' , which are pieces of land connected to a mainland, by a very narrow neck of land named an 'Isthmus'.

THe Word 'peninsula' means 'nearly insular/island'.

However, such is physical geography, and island can becomes connected to the mainland, when tides(sea movement up/down) become low and allow passage by foot, rather than a boat.