An experiment is used for teaching the fundamental statistical concepts: the t-test and the interpretation of the type I error probability  From the results of the experiment, a simple estimator of  is obtained, which gives the students a clear interpretation of the probability of the type I error  “for a given investigation reporting  and hence concluding a significant treatment effect, means that if the treatment had no effect, then there is less than a 5 percent chance of getting a significant value of the t-test by pure chance”. This teaching material is conceived to be presented within the context of an introductory statistics course for students in diverse areas of knowledge.