From THE NEWSLETTER OF THE PUERTO RICO / UNITED KINGDOM CHAMBER OF COMMERCE (bet you didn't know we had one)
I think they drill holes in the balls and for whatever reason the mice fill them with hay/grass and make a new home. I wonder what would happen should a dog get confused between his trusty 'dribble-buster' and a mouse-homes...
thats pretty cool. i love rodents but none of my wee creatures could ever fit in a tennis ball , they must be tiny