One of the most iconic prank viruses of the 80s. Written for Amstrad CPC and later ported to MS-DOS, it did nothing destructive: it just made every character on the screen slowly fall to the bottom like grains of sand. Terrifying for the user, harmless in reality. Here is a pure HTML/JavaScript recreation.
A blue MS-DOS screen will fill the browser and characters will start to fall. Nothing real happens on your computer.
On text-mode DOS/CPC, the screen was a matrix of 25×80 characters stored in a specific memory region (0xB800:0000 for CGA). The virus hooked the timer interrupt and, every N ticks, picked a random character, moved it one row down, cleared its old position. Total <100 bytes of x86 assembly.
Prank viruses of the 80s (DROP, Cascade, LEHIGH, Ambulance) were digital pranks passed via floppy disks. Modern equivalents (ransomware, spyware) are destructive by design. The DROP is preserved as an example of the pre-commercial "phase 1" of computer viruses.
No file on your computer was harmed. It was all HTML + JavaScript in your browser.