In contemporary informal Estonian, the negative verb form ei tea ‘don't know’ has become a routinized part of generic questions, in which the agent is left unexpressed. This pattern is in accordance with the general impersonal and reference-avoiding style of conversing in Estonia. The study outlines a continuum of synchronic usages from the original expressions sa ei tea ‘you don't know’ and ma ei tea ‘I don't know’ to the epistemic usages of (ei) tea, which are specifically tied to the speech act of questioning. The data is interactional and the analysis relies on the interpretation of (ei) tea-questions by the participants themselves, following the methodology of conversation analysis. It is demonstrated that the development of (ei) tea displays phonological and semantic erosion, pragmatic strengthening, subjectification, and decategorialization. Thus, grammaticalization theory is here combined with interactional linguistics in order to display the emergence of a grammatical structure from a discourse pattern.