A Tunneling Engineer Ask’s His Boss for a Raise (Riddle)

David, a very good tunneling engineer ask his boss Rob for a raise.

Rob stops and thinks for a minute, then asks David, “how much do you want?”.

David says, “I want a 20% raise!”

Rob smiles and says, “OK David, I know you deserve it; I’ll give you the 20% raise if you can answer this question in 3 seconds.”

Question: “The more you take, the more you leave behind. What am I?”

3…..2…..1…..TIMES UP!