Programming languages are not only useful to command computers, they also increasingly are a medium for human communication. I will use the framework of distributed cognition to discuss how knowledge is shared in a team of programmers and to show that computer code plays an important role in it. The resulting model of how programmers comprehend code suggests that common grounds play an important role in it. I propose two hypotheses concerning the means used by programmers to refer to common grounds from within their code. The hypotheses imply that modern languages, such as Scala, offer advantages as human communication mediums. I describe an experiment, using an eye-tracking device, that measures the performance of code comprehension. The hypotheses are tested by varying the degree of reference to common grounds.