Stagnate

Flowing water does not stagnate.

The project began to stagnate without effective leadership.

If the water stagnates, algae will grow.

Air stagnates in a closed room.

Ready-witted tenderness […] never stagnates in vain lamentations while there is any room for hope.

Listening to what others say, especially to what they teach, prevents our minds stagnating, thus promoting mental growth into old age.