Problem solving is one of the main competences we seek to teach students at school for use in their future lives. However, when dealing with mathematical problems, teachers encounter a wide variety of difficulties. To foster students' problem-solving skills, the authors developed "strategy keys." Strategy keys can serve as material to foster problem-solving skills. Strategy keys are cards in the shape of keys. On each key is written one general heuristic. To give each heuristic an individual "face," a different key shape for each key and a matching illustration are provided. Also, the wording of each heuristic is formulated to be as simple as possible to avoid difficulties in encoding each key. Instead, students should see a key and associate something with it. This way, the keys prompt new ideas and allow more "opportunities for delving more deeply into understanding the mathematical structure of problems…instead of simply seeking correct solutions" (NCTM 2014, p. 48). Altogether, the strategy keys can be easily integrated into regular math lessons. They provide time-saving materials to prompt problem solving in the classroom. Teachers report positive experiences when using the keys in their lessons, and students' problem-solving processes seem to profit in various ways from the integration of the keys.