Invisible Essay

"Japan ninja student gets top marks for writing essay in invisible ink":

Ms Haga, 19, soaked soybeans overnight, then crushed them before squeezing them in a cloth.

She then mixed the soybean extract with water - spending two hours to get the concentration right - before writing her essay with a fine brush on "washi" (thin Japanese paper).

Once her words had dried, they became invisible. But, to ensure her professor didn't put the essay in the bin, she left a note in normal ink saying "heat the paper".