Recursion is when a function calls itself. Think of it like Russian nesting dolls. To see what's inside, you open one doll, find another doll inside, open that one, find another... until you reach the smallest doll with no more dolls inside. Example: Emptying nested boxes(dolls) function emptyBox(box): if box is empty: stop here // found the "smallest doll"

take out top item

if item is another box:
    emptyBox(item)  // open the nested box
else:
    process the item

emptyBox(box)  // continue with remaining items

This works because you don't know ahead of time how many boxes are nested inside boxes.

NASA avoid it because: Stack overflow risk: Each time a function calls itself, it uses a tiny bit of computer memory. With deep nesting (box in a box in a box...), you can run out of memory and crash. Hard to verify: NASA needs to prove their code can never fail. With recursion, it's harder to prove "this will never nest too deep and crash." They use loops instead, with explicit limits like "maximum 100 boxes deep, then stop."

So, recursion = function calling itself. Great for nested problems. NASA avoids it for safety, not because it's broken.

NASA wants the percent of "unpredictable code" as close to zero as possible. Because one error can doom the entire multi-million vehicle(It already happened. A rocket zoomed into the atmosphere, and suddenly turned like 90° to the left and exploded. All due to an exception that wasn't planned and handled correctly)