As a teacher, I can imagine this played out something like this: "Okay, so we know we can take two numbers and find the parts of them that combine to make 10, then add on the remainder to find the total sum..."

Then, as a class we go through multiple examples because that explanation helped none of them. We painstakingly work in multiple modalities using visuals, manipulatives, verbal explanations, and even songs to teach the concept. The kids who are good at math are bored because they already knew how to do this and you have to find something more challenging for them to work on. If you don't, they instantly start acting out and become a real pain in the ass. The average kids are generally getting it, but they still struggle. The low kids are really struggling and either use learned helplessness and give you a 1000 yard stare, or they start acting out like the high kids to try to not have to do anything. You have to hack through all their bullshit to figure out where the bottom is because JESUS CHRIST HOW ARE YOU NOT GETTING THIS IT'S JUST ADDING TWO NUMBERS TOGETHER AND THIS IS BASE 10 IT'S FUNDAMENTAL TO OUR CIVILIZATION... and then you collect yourself and say "okay, let's get the unifix cubes out and start back at how we add on from the larger number..." because that's what you do. You give the kids what they need no matter what.

Then, after working so hard to help the kids during the day it's 3:45pm, you've been working non-stop since 7:00am, but you need to make a quiz for the kids to take so you make something before stuffing some work in your bag to take home because grading never stops.

The next day you give the quiz,and then that night smug assholes on the internet shit all over you.