Hi Chanz.
This is an age when they really start learning to play, and can be a lot of fun.
Tummy time for strengthening head, back and tummy is very important for learning to sit. A play mat helps for this, as well as placing toys in front of them or in their hands. Most will go to their mouths as they have also discovered their mouths by now.
Story time is also a favourite, using colourful pictures, and reading to them in excited voices.
I also got K a bumbo chair at 4 months and the food tray and put toys on it so that she could play there.
And also a bouncy chair with toys hanging from it which was good fun.
They are taking in and learning so much, so things that engage them and keep them learning for 10-15 minutes at a time is good. They dont have much more concentration than that.
They are very talkative at this age, so interaction with mommy & daddy are invaluable now. Lots of faces, tickle time etc and they'll be laughing like crazy.
Its amazing how quickly you forget hey!
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Take a look at this little one having a blast on youtube!