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keep them on their toes
= If you say that someone or something keeps you on your toes, you mean that they cause you to remain alert and ready for anything that might happen.
@Acer
: to cause someone to be alert and prepared to deal with problems
"The boss made regular inspections in order to keep employees on their toes."
https://www.merriam-webster.com/dictionary/keep%20(someone)%20on%20his/her%20toes