The current study aims to explore how children explain the concepts of biology and how biological knowledge develops across ages by focusing on the structure and functions of the digestive system. The study was conducted with 60 children. The data were collected through the interviews conducted within a think-aloud protocol. The interview data revealed that 5-year-olds think that the digestive system before the stomach and the stomach is a large empty space. Results similar to those found for 5-year-olds were also found for 7-year-olds. The children in the 10-year age group were able to offer comprehensive explanations on the digestive system and 10 children from this age group stated that while some of the food eaten is discharged from the body, some of it is retained in the body. While children from all the age groups develop thoughts on the basis of vitalistic thinking, intuitive and animistic thinking patterns were observed in some of the 5-year-olds. Within the scope of the obtained results, it is thought that carrying out the biology education given at the elementary education level through the supporting methods and materials of the vitalistic thinking will contribute to the understanding of the biology and raising the interest of the children on the topic. In addition, teachers' usage of physical materials in teaching processes could be useful in order to abolish the limitation of animistic thinking on the acquisition of biological knowledge in early childhood.