20 thinking questions is a series of 15 resource books created for teachers who wish to include investigative problem solving in their mathematics curriculum. Each of the 20 thinking questions books focuses on students’ use of manipulatives to solve problems having multiple solutions or solution methods. As part of the solution process, students write about their thinking prior to engaging in classroom discussions. Working with manipulatives gives students an opportunity to explore abstract concepts while manipulating concrete objects. Writing about one’s thinking challenges students to clarify their ideas and provides teachers with insights into each students thinking and level of understanding.