Units!
“Units are life or death!” I tell my students. I often use the following example:
Let’s say we’re all on a boat and Mary falls overboard. I turn to you and yell, “Quick, get me 12 of rope or Mary will die.” You bring me 12 centimeters but I needed 12 feet and alas, glug, glug, gurgle, gasp, poor Mary is dead. Units are important.