In the same way that FOOT and BOOT are two different words with different meanings, also the words SHEEP and SLEEP have different meanings as well. A SHEEP is an animal, while SLEEP is what you do when you're tired. Just changing the letter H to the letter L makes a huge difference.
Another example of this is WEDDING and BEDDING. WEDDING is a ceremony in which a couple is united in marriage, whiles BEDDING is what you out on your bed, like sheets and things. The W is changed to a B and changes the meaning of the whole word