My Mum and my Nan are working together on a project to knit a blanket each for the two youngest in my family. To make it easier for themselves, they are knitting lots of 5 inch squares to sew together to make a patchwork blanket. Mum asked Nan to work out how many squares they will need to make a blanket roughly the same size as our table top. She did the right thing by counting how many squares it was down the table, and how many across. It was 12 by 7.
Now, when you multiply the length of a rectangle by its width, you get the area, so they needed 84 squares to make a blanket the size they wanted. They were both happy with this. However, in a fit of panic, Nan phoned up Mum a few days later.
"I didn't count the middle squares! We are going to need a lot more than 84!"
I still don't think Mum has told her that she didn't only work out the perimeter...
Mathematics - it can save you from knitting a much larger blanket than you planned!