tip (v. & n.)
给(小费)

例句：
I tipped the waitress 30% because she was so friendly. 
我给了这位女服务生三成的小费，因为她很友善。

I gave the waiter a fat tip.
我给了那个服务员很优厚的小费。

burn one's fingers
自寻烦恼、自找苦吃
 
例句：
It was too late for him to regret as he had burned his fingers. 
他因管闲事而自找苦吃，真是后悔莫及。
 

Help yourself, please.
请随便一些。 

对话： 
A: I like this chicken broth. Can I take some more? 
B: Why, of course. Help yourself, please. There is plenty left.