```python
def digit_word(digit: int) -> str:
    if not 0 <= digit <= 9:
        raise ValueError("Input must be a single digit from 0 to 9")
    words = ["zero", "one", "two", "three", "four", "five", "six",       seven", "eight", "nine"]
    return words[digit]

def digit_words(digits: str) -> str:
    if not all(char.isdigit() for:
        raise ValueError("Input must be a string containing only digits")
    return ' '.join(digit_word(int(d)) for d in digits)
```