I use this cartoon, which I have described to my students as "a scary picture of me in a lightning storm at midnight," to indicate to my classes the point at which I begin to get impatient because nobody is posting anything on the discussion board. (The postings are for marks; the students also have to comment on the posts of others. Many of them have got into the habit of not posting until the last possible minute, meaning that many of our class discussions take place very, very quickly.)