As you know we have Poppy a 9 month old Westie and Pickles a nine week old Border Terrier x Jack Russell and yes they are proving to be a handful.  However, can you tell me why Poppy constantly smells Pickles' underneath, why when she is playing she tries to bit and also why when they are playing Poppy lies on her back with belly exposed. 
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At times I cannot tell if they are playing and if they are fighting.  As Poppy seems chase Pickles in an aggressive manner.

Please help!!!

They are both puppies and they are both terriers and so play will be rough. :) I'm sure you would notice the difference if they actually were fighting as it is a huge difference.  My pups play extremely rough as well, growling and snarling and biting and chasing. :) If Poppy sniffs Pickles underneath all the time, you may want to check out Pickles hasn't got any vaginal infection as that COULD be a reason (and it's fairly common in pups), but equally she could just be checking out what sex she is -at 9 weeks I assume you haven't had Pickles very long yet. :) Laying on the back can mean the dog is submissive to the other one, but equally pups often play like this. My 2 year old Golden would go outside, flop on his back and have a litter of 8 malinois pups all jump on him and he LOVED it, LOL.

All sounds pretty normal to me. :)