Pupilla muscorum is a small snail with a shell width about 1.6 mm and with a shell height about 3.5 mm. It occur in many countries in the Northern hemisphere: USA, in Eurasia from nearly whole Europe to Pakistan.
Although it is widely distributed, malacologists usually know this species a shell only like this:
When you will find it and collect it, the live snail is crawling barely although it occur in dry habitats. I think that every snail of such size requires some moisture for crawling. I put the shell into a drop of water and the snail peeked out. Then I taken these photos. You can repeat it easily to observe this interesting small animal by yourself.