Play a pair of fives according to the strategy for a hard total of ten.
Almost all basic strategy assumptions begin with the premise that you'll be dealt a ten when you take a hit. By splitting fives, you take a good hand (hard 10), and turn it into two stiff hands.
A total of 10 is good because there are 20 cards in a 52 card deck that will give you a total of either 20 or 21. Either of those totals is excellent.