Context: Developers possess mental models containing information far beyond what is explicitly captured in the source code. Objectives: We investigate the possibility to use source code annotations to capture parts of the developers' mental models and later reuse them by other programmers during program comprehension and maintenance. Method: We performed two studies and a controlled experiment. Results: Developers' mental models overlap and thus can be shared. Possible use cases of shared annotations are hypotheses confirmation, feature location, obtaining new knowledge, finding relationships and maintenance notes. In the experiment, the presence of annotations reduced program comprehension and maintenance time by 34%. Conclusion: Annotations are a viable way to share programmers' thoughts.