Caesar Cypher--- I don't understand why it doesn't work

[ironsheep]

Thank you for the clarification--- I don't understand why the computer doesn't understand what I meant with cleartext_lowercase(). You understood what I was trying to do, how come computers can't?? What is with the computer and it's thinking??

alphabet= "abcdefghijklmnopqrstuvwxyz"
cleartext= input()
cleartext=cleartext.lower()
def encrypt(cleartext):
    codedtext = " "
    for character in cleartext:
            if character in alphabet:
                newcharacter= (alphabet.find(character) + 13) %26
                codedtext += alphabet[newcharacter]
            else:
                codedtext += character
            return codedtext
print(encrypt(cleartext))

Now when i run the program with sheep as my input, all I get is f---- the other letters are not showing up. f goes with s, but what about heep?? This should be working because I have codedtext+=alphabet[newcharacter].