“Two contributors to explicitness are the observability of the act and the unequivocality of the act. Some acts are not observable and we may know them only by inference from assumed consequences. You leave a dollar bill on a checkout counter, turn away for a moment, then find it missing. The consequence is obvious, but do you know if the customer next to you took it or if it was carried away by a draft from the open door? Acts themselves can be equivocal, forgotten, or otherwise intractable.”