Today I rearranged the puppy pen to include a crate.  The puppies had been sleeping on towels in the corner of the pen but after breakfast this morning, they discovered a new "den" with bedding that smelled like "Momma".  They quickly went inside and fell fast asleep (notice that there is no door on their crate.  Now, they run out to potty and go back in to sleep. I find that by adding a crate as part of their environment as babies, they learn very early that a crate is a warm, cozy place to be. They also learn the difference between being inside the crate where they sleep and outside the crate where they potty.  It's so easy to transition them when it's time for house training.