If you can't answer the question, but you know the expert than can, you can refer the question to them.

1. Press the "+" symbol at the bottom of the page
2. Press "Refer"
3. Begin typing a user in the search box
4. Select the user(s) you want to refer the question to
5. Add a description so the user(s) know why you are referring the question to them
6. Press "Send" to notify the user(s) that a question has been referred to them