If you see this, please pat yourself on the back and award yourself 1 sleeve rolling point. Also, I don't like rolling up my sleeves, it's all a lie.

Try this:

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Hi, this is the first time I have commented but for a taster day we use phenolphtalein on the paper for the writing and use 0.1M sodium carbonate solution to show the message. works everytime as nice pink letters.
Hi the previous tech has made a pink sol of Sodium hydroxide, water and phenolphthalein and has done an activity where they squirt this on to a skeletons lab coat.

Would anyone be able to advise on how much of each in that solution/molarity as HOD Iis keen to do it
Fool proof method we use for open night is cobolt chloride solution 0.1M use paint brushes to "paint" this on pink paper and then place on a hot plate on a low heat and it goes blue. Works so easily each time
There's a link I was looking at the other day, although I've not tried any.

I have done lemonjuice with my cubs and a tea light in a jam jar - so tey can't get the paper too close to the flame.

In WWII spies wwre trained to use semen - just saying lol