As Penny has become older she has become less interested in running around with other dogs. She seems to prefer people. Now that I've read an article saying it's possible to work out dogs' friendliness by observing which side they wag their tail on, I'm intending to watch her carefully next time we meet a strange dog. I'll have a look at the other dog's tail-wagging, also, to see whether I can work out which side they prefer.
For those with a scientific bent, there's a fun site at ScienceBuddies that explains how to conduct your own experiment into tail-wagging.
At the Pets site there's an extract from Dr Stanley Coren's book How To Speak Dog, explaining when and why puppies start to wag their tails and how dogs use tails as communication devices, and also for balance.
I've often seen Penny use her tail as a rudder when she's changing direction in the water. I've also noticed she holds her tail low when she's concentrating or focusing on an object of interest. (Like those pesky rats in our back yard that she hasn't managed to catch yet.)