Keep someone on their toes
To keep someone on their toes means to force someone to stay active, alart, and focused on someone or something.
Examples:
My kids always keep me on my toes. They're so unpredictable, So they always keep me alart and ready for anything that might happen.
Your teacher might keep you on your toes during a class. He can ask you a lot of questions, he can nominate different students in random order, he can give you some tests, So this way he keeps you on your toes. he makes sure you are focused for the whole time.
