Estimation is one of the hardest skills that I have to teach. My students firmly believe estimation means to count. That is not it at all. I want them to use the information they know to predict a great answer. NOT just GUESS!
We do this cookie activity to introduce estimation. Each student gets a cookie. I began by rambling off that there are 1 billion chocolate chips in 1 cookie, 300, 295. Of course the students are like no way. They have a pretty great understanding of spatial sense. I then introduce the term estimation and we discuss. The way we use prior knowledge with this activity is by counting the chocolate chips on top of the cookie, therefore we know that it has to be more then that number. I have the student write their number on one side of the paper towel with crayons. Another lesson inside of that is because they feel as when their answer is wrong they can erase it and change it AFTER we count.