Answer: A jar of acid
During the occupation of Denmark by the Nazi regime in the 1
The Hevesy knew that there was a ban on the export of precious metals from Denmark and that if one of the men tried their medals with them upon leaving the country, they could be prosecuted and imprisoned or put at risk.
To hide the gold medals in such a way that the Nazis would never find them, the Hevesy dissolved the two medals in nitro hydrochloric acid (a very powerful acid that can dissolve gold and platinum). He placed the resulting solution on a shelf with other chemicals in his laboratory at the Niels Bohr Institute, where it seemed harmless enough, hardly valuable and not the kind of solution that someone would want to play with.
When he returned to the institution after the war, he found the solution completely undisturbed. After specifying the solution to extract a pile of gold powder, he brought the gold to the Nobel Society where they rearranged the medals using the original gold that was so carefully and cleverly hidden by the Hevesy.