Who wants surprises, anyway? I do. Maybe you do, too.
In English, we often omit words rather than repeating them. This is called 'ellipsis'. Ellipsis is often used in replies.
When you reply to wh- questions, you can often use one word or a group of words rather than a full sentence, in order to avoid repetition.
Example: 'Where are you going for your holiday?'
('I am going to Germany for my holiday' is correct, but it sounds redundant.)
Short answers must be accompanied by an auxiliary verb.
Example: 'May I buy you lunch today?'
'Yes, you may.'
When there is no auxiliary verb, you must use 'do', 'does' or 'did' (see example at the top).
Example: 'Who broke the window?'