Twitter question for direct messaging?

[Kardashian]

I'm following someone on twitter, but they are not following me, However, they give me a direct message in my direct messages, but how come I cannot reply back to them as a direct message?

[Ian Sheppard]

You can't. You can only mention them

[Daniel]

Its Because They Are Not Following you Thats Why you Can Not Reply Back in order For Both of you to Write Direct Messages to Each Other you Both have to Be Following Each Other