Bring in a large cylindrical glass jar that is filled with marbles of different shapes and sizes. Also bring in a tiny cylindrical glass jar that is filled with marbles. Tell the students the amount of marbles that are in the tiny jar.
Their task is to estimate the number of marbles in the large glass jar. At the end of the activity, have students count the marbles in the large glass jar and compare them to the mental math answers they came up with.