Cost for doing research

When I do my freelance service, sometimes client wants me to do something that I never do before. Lets say a client wants me to create web application that can update his twitter account automatically.

Since I have a basic of programming language, obviously this is something that I'm capable of doing. However, it's just I never have a chance to do it before. So, before I can implement that, I have to do some research.

When you do some research on a request, how do you say that to your client? Do you mention it on your invoice? Or do you just include it in hourly rate?