I saw those kids sneaking around the back of the store; they must be up to no good.

我看到那些孩子在商店後面鬼鬼祟祟的；他們一定在搞什麼壞事。

My cat has been unusually quiet all morning. I bet she's up to no good.

我的貓整個早上都異常安靜。我敢打賭它在搞什麼壞事。