This interactive applet helps develop number sense. The user mentally estimates a number that is represented by an arrow on a number line and then checks the estimate by clicking to have the exact number revealed. Users can choose a number range for whole numbers (between 0 and 10, 100, 1000 or 10,000) or decimals (tenths or hundredths between 0 and 1). An optional scale of tick marks provides guidance. The activity does not provide a scoring component.