Recognize that in a multi-digit number, a digit in one place represents 10 times as much as it represents in the place to its right and 1/10 of what it represents in the place to its left.
Imagine a world with no hundreds place. We'd have to call it ten tens instead. But then, what would we call the thousands place? How would we read 9999? What if we added one more?
Place value is something we cover in elementary school. It seems simple, but I'd wager that very few adults really understand the topic. I sure didn't until I worked with non-base-10 number systems in college. Your students can get a taste of this mind-boggling experience by imagining what it would be like if we didn't have the number 9. What would each digit represent then?