A: How about going to the beach this weekend?

B: Sorry, but I can’t. I have to pull in some overtime.

A: Why?

B: Our new product has sold well in Asia. But the European market is a tougher nut to crack. The boss has asked me to make some changes to the design.

Note: “A tough nut to crack” means a person or thing that presents difficulties. Just imagine a dry fruit consisting of an edible kernel or meat enclosed in a woody or leathery shell. It’s not easy to crack the nut and eat its kernel. In Chinese, people would call someone hard to impress or some difficult task “a hard bone to chew on.”