This is a very mini lesson that can be used on a daily basis and expanded upon during the math time.
Use a clear jar (I use a clean mayonaise jar) and fill it with edible items; for instance, candy, animal cookies, pretzels, etc. Be sure to count the items as you place them into the jar. (I usually write the number on a piece of paper and tape it inside the lid.) Ask the students to estimate how many items are in the jar. They may write their guesses down on paper and place them in a separate container for you to check later. The next morning, write on the board the number is greater than ___ and less than ___. This teaches them the greater than less than signs as well as understanding that a small jar can not hole 4,000 items.
When a student guesses the correct total, he gets to take the items home; after they share at least one each per classmate.
This can be used over and over again and it never gets old. As you teach the estimation lessons, their ability increases.