Or you could say, "Mum, I'll show you. cross your hands behind your back and..."
Sorry, I couldn't resist that one.
Actually you don't have to worry at all. If she doesn't open it -- fine. if she does, say the truth in a light most favorable to you: it's stuff for a an idea of a project I'm working on." Quite truthfully, you don't know any more than that until you are pealing the tape off for real, do you?
Have fun, play safe, play often.......