Math Professor sends his wife a text message. "Dear, you 54 yrs old, and can no longer satisfy my needs. When you get this I'll be at a hotel with my 18 yr old assistant. I am sorry, but I will be home late". Wife sends reply "Dear, you're also 54 yrs old and by the time you get this I'll also be at a hotel with the 18 yr old pool boy. Your a brilliant Mathematician so you will know that 18 goes into 54 a lot more times than 54 into 18 does. Don't wait up"