Curiosity is a 21st century skill and is of paramount importance in the digital age. However, the assessment of curiosity is often based on self-report or subjective observations. We present the development and evaluation of a digital quantitative assessment game for question-asking-based exploration. The student navigates a graphically presented question graph by selecting questions about a series of virtual alien worlds. The game extracts question-related quantitative measures, e.g., the breadth, depth and specificity of the answers to the questions. We conducted a study with Youth University students and administered a curiosity-based questionnaire to their class teachers as an external validation. Our results show that the measure of total question specificity in the last presented world is a significant predictor of children’s curiosity, as rated by their teachers. This suggests that curiosity can be quantitatively assessed by an entertaining digital question-based game.