顺手牵羊
Meaning
To take something opportunistically while doing something else — often petty theft.
In a sentence
He went for a coffee and 顺手牵羊 took a pen from the desk.
Usage note
Negative. Do not use it for harmless convenience.
Where you see it in Brain Challenge
This idiom appears in the picture-idiom levels as a picture or number clue. When the clue shows numbers, the digits usually become the first characters of each half of the idiom — that is why so many chengyu start with 一, 三, 五, 七, 九 or 十.