Strike While the Iron Is Hot

To act at the right moment when circumstances are favorable

Fit as a Fiddle

To be in very good physical condition

Teacher's Pet

A student who is the teacher's favorite

It Is Raining Cats and Dogs

It is raining very heavily

Cry over Spilt Milk

To be upset about something that has already happened and cannot be changed

Sky Is the Limit

There is no upper limit to what can be achieved