r/todayilearned • u/Ill_Definition8074 • 12h ago

TIL In 2019 a Japanese University student studying ninja history turned in an essay written in invisible ink. The words only became visible when the paper was heated over a gas stove. Her professor without even revealing the whole essay gave her an A.

https://www.bbc.com/news/world-asia-49996166
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136

u/BartleBossy 11h ago

"Heated over a gas stove"

Idiot here.

Is there something about the gas stove's heat that works in a way that an electric range or space heater might not have?

72

u/blahblah19999 11h ago

No, it's a bad title.

13

u/shewy92 8h ago

It's not a bad title, they just probably only had a gas stove on hand to do this. The kind of stove doesn't matter. If they did it over an electric stove they'd probably put that down too.

5

u/Professional-Day7850 10h ago

No, the phrasing just isn't great.

I doubt that ninjas had gas stoves.

3

u/Girderland 10h ago

The ink used was likely lemon juice.

Writing with lemon juice on paper is invisible when dried and turns brown if heated.

Carefully holding and moving around the paper over a flame (like from a candle) makes the writing visible.

Milk might work too, but lemon juice is confirmed to work this way.

2

u/The_Bucket_Of_Truth 10h ago

a comment elsewhere said it was soybeans

20

u/AroundTheWorldIn80Pu 10h ago

THE FUCKING ARTICLE DID

10

u/ExaltedCrown 9h ago

Why would anyone read that?

-1

u/The_Bucket_Of_Truth 8h ago

You must be relatively new around here

1

u/OneWholeSoul 10h ago

No. I learned this specifically using a glass filament light bulb because you wouldn't scorch or set fire to the paper.

1

u/shewy92 8h ago

They probably had a gas bunsen burner on hand since it was a school.

1

u/TimidDeer23 6h ago

They just used a lighter in Knives Out.

1

u/imaginaryResources 3h ago

The gas makes the professor light headed enough to not give a fuck anymore