Building skyscrapers of numbers.
Multiplying large numbers using the standard algorithm.
Requires keeping your columns perfectly straight.
Don't forget the 'magic zero' placeholder on the second line!
Plain English: When multiplying big numbers (like 24 x 12), we multiply the top number by the Ones place, then by the Tens place. Because it's the Tens place, we put a Zero placeholder on the second line.
Real-world example: Calculating the total pixels in an image (Width x Height).
Forgetting the 'Magic Zero' on the second row. It ruins everything.