5 months 4 weeks old

Your baby's development is now working in your favor when He's fussy: He's increasingly distractible. Intensely interested in the world around him, his rapidly evolving brain zeroes in on things that are new and different. So when your baby is a little cranky, use his growing interest in novelty to your advantage by making a funny noise, singing a song, gently banging a pot - any change from the norm may snap his out of her grumpy mood.

From: Baby Center