why can't you find 1 when you need 1

Well iv just been out to my scoobie to check the power steering belt i removed the top cover that covers the altinater and pump i pressed on belt mmm seems ok might need a slight tightin let check alt belt while im here press by god thats loose defo needs a adjust so in the house i go not sure what size i need a got all sockets i have to find its a 12mm and guess what i dont have a 12mm iv gor a 11mm and a 13mm so iv had a ring a mate to see if he can lend me a 12mm arhhhhh