Handling babies in the box. I take them out twice a day and give them a basic check. Bellies full, hair growing, looking healthy. Day three I give them a good going over as I often find I can tell which one (if any) that I want to keep for myself.
Starting around day nine I start handling them more. Switching them from hand to hand, holding them in various position, light pets, etc. I play with feet a bit, but not over much. Just enough they know I'm there are are ALERT to me handling them. At this stage I watch for babies that startle easily and give them MORE handling, making note that I won't be holding them back and will aim for teen/adult homes for those kits. At this stage I watch how their eyes are doing as well, start checking teeth etc.