def big_number(int_in):
    """Converts a potentially big number into a lisible string.

    Example:
    - big_number(10000000) returns '10 000 000'.
    """

    s = str(int_in)
    position = len(s)
    counter = 0
    out = ''
    while position != 0:
        counter += 1
        position -= 1
        out = s[position] + out
        if counter % 3 == 0 and position != 0:
            out = " " + out
    return (out)