```python
def long_word(sentence: str) -> str:
    # Split the sentence into words based on spaces
    words = sentence.split()
    
    if not words:  # Check if there are no words in the sentence
        return ""
    
    longest_word = words[0]  # Initialize with the first word
    
    for word in words:
        if len(word) > len(longest_word):
            longest_word = word
    
    return longest_word
```