The store happened to be crowded yesterday

The explanation may be much more complex

This might lead us to believe that a simple culture would make use of a simple language, that a complex culture would make use of a complex language, and so on

What seems simple to you seems complex to me

He came up with a terrific solution to the complex problem

The situation has evolved into a more complex problem

The next train will be less crowded than this one

This story is too complex for children

I was walking along the crowded street when all at once I heard a shrill cry

The roads out in the country are not so busy as in the city