Common mode failure

Okay kiddo, have you ever played with building blocks or LEGOs? Imagine you have two houses made of blocks that are exactly the same. Now, let's say that each house has a door that you need to open to get inside. If one of the doors breaks, you can still get into the other house, right?

Now, let's imagine that the two houses are connected by a bridge made of blocks. If one of the blocks on the bridge breaks, you won't be able to cross the bridge anymore, even if both of the houses' doors are still working.

Common mode failure is kind of like when one block breaks on the bridge and it stops you from being able to get into either house. In technical terms, it's when two systems or parts of a system fail at the same time, which causes the whole system to fail.

So, just like how the broken block on the bridge stops you from being able to get to either house, a common mode failure can stop a machine or system from working altogether. Engineers and designers have to be very careful to prevent common mode failure in important systems like airplanes, cars, and medical equipment.