Lemon juice secret writing

Writing a secret message in lemon juice, then revealing it with heat, is a neat gimmick, but I ran into a few practical problems when I used it in a game, so here are the lessons learned. I was doing this on cards from a shopping centre "print your own cards" machine - so this advice has only been tested on thin card.

• Squeeze out a couple of lemons, or use ready-made lemon juice from a container (either works), then write your message with a cocktail stick.

• Write thickly - you need to redip the stick every couple of strokes. Then blot and leave to dry - this procedure left no visible trace on the cards.

• Revealing the message needs something HOT: ironing the card works best. Holding it right next to a conventional lightbulb is a useful backup, but smokier.

• A travel iron may not get hot enough.

• Till receipts go black when ironed, so don't use those.

• I did the message the day before and stored cards between notebook pages and in the pocket of a bag. I don't know how long the message will 'keep' before going off.