Classroom teachers of Kindergarten, 1st Grade, or 2nd Grade often use the Hundreds Chart puzzles to teach children the basic mathematical knowledge and skills like counting. Computer programs have been developed to help them to create puzzle sheets by cutting out the numbers, but students still need to solve the puzzles on the printed paper. This study designs and implements a computer application named Hundred Acorn Forest as an instructional method to teach basic counting skills with the Hundreds Chart puzzles using the Unified Process method. The application not only offers an easy way for classroom teachers to create and edit puzzles based on patterns, but also engages students in a unique interactive game play environment, which helps to maintain their interests of learning. In addition, the video playback that demonstrates the process of students solving the puzzle provides classroom teachers valuable information to set up levels of control and consistency that have not been available before.