[R] Could anyone tell me how to open .RNW file?

[F Duan]

Dear R people,

I find some of help files under doc directory in R are RNW extensions. Could 
anyone explain to me what they mean and which program I should use to open them?

I tried to open them by Rgui.exe but failed. Currently I use wordpad (or other 
txt editors) to open them though the opened files are not very readable.

Thank you.

Best regards,

Frank