She’s going

Now that mum actually knows she has to go, she feels better, although she’s never going to get better and that is why she has to go.

 Mum doesn’t know where she is going, but she thinks it might be nice.

 We all think we know where we are going, but we have never actually met anyone who has been there.

 I tell her to send me a postcard. On this journey at least, there’ll be no danger of the postcard arriving after mum gets back.