Is it Rice? Is it Flax? Is it…?

That was the question I was asked when I changed the sand from
                                 our new structure.
               
What do you think it is?
               
It flows through the holes and funnels easily.
     
It makes a soothing sound when it hits the tin cans, plastic funnels or
wicker basket.  It sounds like a rain stick.

            
Have you figured it out?

It's thistle seed, for the birds.