The cache only has a limited size. To make room for the previously uncached entry, another cached entry may need to be deleted from the cache. Special rules are used to find the entry that should best be deleted. These rules are usually called Heuristics. Heuristics used to find the entry are called replacement policy. A very simple rule used is called "Least recently used" (or LRU). It simply takes the entry that was used the longest time ago. Other heuristics are listed at cache algorithm..