The current anti-SIDS thinking is that infants should sleep on their backs. However, supine sleep (sleeping on back) is associated with temporarily lower social- and motor-skills development (through 18 months). Infants are to be given “tummy time” to avoid developing into a socially-inept, fumbling person, and to practice learning to lift their heads and crawl around.
Today was Ethan’s first “tummy time”: