PLUS that is what makes it gourmet. It is something she only gets to do once in a rare while, AND its super cool.
If you aren't aware of the so-called secret. Just dip a toothpick in lemon juice (as if you are using a quill and ink) and write with the juice. You'll need to re-dip often. After the juice dries (unless you used a whole bunch), you won't be able to read the message until after it has been heated. Our hair drier doesn't get hot enough so we had to do the heating part with candles. Keep the heat source moving, but moving slowly in comparison to the paper (or the paper in comparison to the heat source). Wherever there was lemon juice will brown faster than the paper around it revealing your "secret message".