Relationship of space and argument

Let's see the following code.

def times_two(arg1);
  puts arg1 * 2;
end

def sum(arg1, arg2);
  puts arg1 + arg2;
end

times_two 5
times_two(5)
times_two (5)
sum 1, 2
sum(1, 2)
sum (1, 2)

when call the functions above, which returns error?

Result...

10
10
10
3
3
SyntaxError: (irb):14: syntax error, unexpected ',', expecting ')'
sum (1, 2)
       ^

Why??

Because there is a space between function and parenthesis of arguments, Ruby parser find it as a single argument, but it is invalid, so returned error.