Irregular Tetrahedra

In Teba's fundus I found a box with a number of irregular tetrahedra (aka triangular pyramids). They are magnetic and the goal is to create different shapes. From easy number 1 through difficult number 20.
I thought I give number 19 a try, but it took me unexpectedly long. I gave up and tried lower numbers. I think I managed most of them, but some I still didn't. Too difficult for me. (And I know I faked some...)
Thanks to Ingeborg for hosting the Abstract Thursday challenge.