There's always problems in answering a door, but what can't you say about about? First is handling the people who joshingly want to know whether the door is asking questions you can't answer. Of course you can answer them; the challenge is wanting to. The question is always ``is there someone on the other side of this door?''
This is why it's important for the door to have sides. If you find a door that's a non-orientable surface you're probably dealing with something deep into topological mathematics. Topology sounds like just lighthearted fun when you're just talking about doughnuts and coffee cups being the same. Later, when you start trying to figure out if you're looking at tangent space bundles, or bundle tangent spaces, or space bundle tangents, you realize how much more pleasant the old-fashioned doors were.
Having doors to answer is a great development in making life more peaceful. Imagine you have to do without doors when someone wants to see you. If you didn't want to see that person, you'd have to rapidly turn invisible before you're noticed to be definitely inside. Even when you did there's the risk that your clothes would be left visible, so people would know you were hiding, or that even if you remembered to make your clothes invisible too you'd be rocking gently in your chair.
Yes, you only do this to pretend to be exercising all day, burning off of nearly a dozen more calories than normal, but it still leaves proof of your presence to the person you didn't want to see. All you've managed is to not be seen. Whereas with a door you can leave it closed and when the person knocks, answer incorrectly. All sorts of hard feelings and suspicious visitors testing the chair by sitting on you are avoided.
It's also a benefit if you want to find someone. Suppose you're extremely shy and need to see a person who hasn't got a door. Then you can't help noticing the person since there's no obstacle. But with the door there's the chance to step up to it, listen for evidence of invisibility on the other side, and then leave without knocking. Everyone can carry on peacefully and no terrible social tensions have to be faced.
The normal way to answer the door is to mean to say, ``Yes?'' or ``Come in'' or ``Who is it'', possibly while opening the door. By mistakenly saying something like ``No'' or ``Wobble'' you can introduce the irreducibly awkward little moment that makes the rest of the day feel like you were going down the stairs with one foot halfway off the steps. It's a challenge to make the interaction not uncomfortable enough to break off all contact, but with effort anyone can do it, really.
You might try responding to knocking by knocking back, in Morse Code, but the only Morse Code you remember is for SOS. So instead tap out short-long-short-short, which means you're sending a lot of letters `L'. This requires whoever's on the other side of the door do a lot of interpreting of your messages, because you had thought short-long-short-short was the Morse `F'.
Given that the question is ``is there someone on the other side of this door'' we're left with the odd conclusion the door isn't enough to convey the door-ness of things. That is, if you just had a door standing somewhere it wouldn't need to be answered, because someone could just look around it and wouldn't have to interact with the door in any way, except when the door fell on him. The essential door-ness trait of needing sometimes to be answered isn't conveyed by the door but by having a wall the door's in. With a wall then a door can achieve its full door potential. However, a door is not needed for a wall to achieve utter wall-ness. So if the door isn't fully door-y without the wall while the wall can be fully wall-ish without the door, then you see whether your original number was divisible by nine. See how that works?
Trivia: In response to the Spanish Influenza pandemic, Prescott, Arizona, made it illegal to shake hands. Source: The Great Influenza: The Story Of The Deadliest Pandemic In History, John M Barry.
Currently Reading: How The World Was One: Beyond The Global Village, Arthur C Clarke.