how to create a function named word count that takes a string and return a dictionary

# E.g. word_count("I am that I am") gets back a dictionary like:
# {'i': 2, 'am': 2, 'that': 1}
# Lowercase the string to make it easier.
# Using .split() on the sentence will give you a list of words.
# In a for loop of that list, you'll have a word that you can
# check for inclusion in the dict (with "if word in dict"-style syntax).
# Or add it to the dict with something like word_dict[word] = 1.
def word_count(my_string):
    string_dict={}
    for wd in my_string.split():

        if wd in string_dict[wd]:
            string_dict[wd]+=1
    else:
        string_dict[wd]=1
    return string_dict