www.brainpop.com/math/practicalmath/estimating
https://pbskids.org/cyberchase/games/glowlas-estimation-contraption
http://www.shodor.org/interactivate/activities/Estimator/
https://www.mathgames.com/estimation
http://studyjams.scholastic.com/studyjams/jams/math/numbers/nestimate-whole-numbers.htm
