Get a notebook. Or use the sound recorder app from your cellphone. Whatever. The important thing is to store your ideas. File them somewhere (choose what is easier for you). Because someday this idea will appear out of nowhere, as happens when you find money in the pocket of an old pants. (I love this feeling… don’t you?)
Ideas come and go all the time and everywhere. It’s up to you organize them. Once you have stored your ideas, it is far more easy to develop and create something different.