Zack and his mule were walking down the road when one of Zack's friends drove up and offered him a ride to town.  Zack got into the truck while his mule ran along behind.  The mule was right in back of them as they reached
55, and stayed with them as they sped up to 70.

"I'm worried about your mule," said the driver.  "his tongue's hanging out."

"Which way?" asked Zack.

"Left," his friend said.

"Well, stay in this lane - he's about to pass."