Ponder This

This wooden puzzle was given to me many years ago by my Danish brother-in-law. It kept me busy for a long time until I finally figured out the secret.

After I recently found this in a tin box of memorabilia, my daughter proceeded to start taking it apart, and immediately found it impossible to put back together. I promised her that it would be whole again when she got up, and was fairly pleased with myself when I remembered the trick to finishing it.

I wonder if every wooden puzzle square has the same trick? Probably not, but it would be fun to find out.