If you are replying to emails or composing a new message inside Yahoo! mail and have trouble deciding words that should go in the subject line, try this fun feature.
Click the "Subject" button (see screencast) and this will auto-fill the subject line of your email with some funny and interesting one-liners. You can continue clicking the Subject button until you stumble upon a line that may be relevant to your message.
To discover more Easter eggs in other Yahoo! products, check the Yahoo! blog.