I recently saw a video on gallium infiltration on YouTube. Gallium is a metal with a melting point somewhere between standard room temperature and human body temperature, meaning if you pick up a block of it sitting on the table, it will melt in your hand. If this melted metal comes into contact with solid aluminum, it will seep into it and alloy with it, decreasing the strength of the aluminum to somewhere around that of tissue paper.
When I saw this, I immediately had a horrifying thought: aren't airplanes made of aluminum? And when I've flown in the past, I've seen a few places where there's exposed metal. This makes me wonder if a malicious person with no regards for their own safety wouldn't be able to bring down a plane with a novelty for children that anyone can buy for a few dollars.
If someone tried to do this, is there anything in the design of modern commercial airliners that would stop them?