Previous research on children's knowledge about the mind has primarily focused on their comprehension of false beliefs, leaving the conceptualization of thoughts and thinking less explored. To address this gap, we developed a new assessment tool, the interview about the mind (IaM), to assess children's understanding of the mind. Two studies involving Polish preschool and early school-aged children were conducted to assess the validity and reliability of the tool: a cross-sectional study with 212 preschoolers aged three to 6 years (106 boys, M = 59 months) followed by a longitudinal study with approximately 200 children (110 boys) assessed at ages 5.5, 6.5, and 7.5 years. We found that the IaM possesses robust psychometric properties and a one-factor structure. Younger children recognize that thoughts are imperceptible to the senses and identified the mind's location as the head. Older children exhibited difficulties with analogical or abstract definitions of thoughts. As expected, a linear developmental trajectory was evident from ages 5.5-7.5, with children's language abilities showing a positive correlation with IaM scores. Future research should explore the relationship between knowledge about the mind and theory of mind (ToM), as well as cross-cultural differences in children's mind conceptualization.