Early scientific thinking in kindergarten (6-year-olds) was investigated in a large study involving 227 participants. We investigated (1) whether individual differences across 3 scientific-thinking components (experimentation, data interpretation, and understanding the nature of science) are stable across children, (2) whether children's increased information-processing skills (intelligence, language abilities) and their development of an advanced theory of mind (AToM) are potential mechanisms that bring about individual differences in scientific thinking, and (3) whether individual differences in scientific thinking predict individual differences in science content knowledge. Using a newly developed instrument with 30 items (the Science-K inventory), we found that a one-dimensional Rasch model provides a good fit to the data, showing that kindergarteners' scientific thinking goes beyond competencies in "single" tasks. Individual differences across the diverse scientific-thinking tasks were stable, and children's scientific thinking was correlated with their intelligence and their language abilities. AToM was an important precursor for scientific thinking, predicting it independently from the influences of information-processing skills. Children's science content knowledge was predicted by their score on the Science-K inventory. This finding points to a specific relation between these 2 components of scientific literacy, and it supports the view that scientific-thinking skills help to better test and revise (wrong) beliefs and misconceptions about science. Our study is the first to reveal substantial individual differences in scientific thinking in children as young as kindergarteners, and it shows that scientific thinking is important for a successful learning of science concepts, even before children enter formal schooling and begin science education.