“Ok, well I wasn’t JUST in China”.
When you bothered eight different people to borrow a pen and fill out that customs declaration form on the plane, it asked you what countries you’d been to. That doesn’t mean customs won’t ask you again to make sure you’re not conveniently “forgetting” that little jog you made to Bangkok. If you’ve got any inconsistencies in there, and then you admit it to customs, congratulations. You’ve just lied to customs. Don’t plan on being anywhere for the next month.
Anything rude, at all.
Even if your flight was delayed, you sat between a fat guy and a screaming baby on the plane, there was no food, and the only on-demand movie it had was “Gigli", don’t take your anger out on the customs agents and get attitude. They have ALL the power, and while most don’t abuse it, getting rude or angry with them is a great way to ensure you spend a lot longer at the airport than you wanted to. Just be polite, concise, and truthful, then save your plane rage for the guy who lost your bag.