Just wondering why my four year old crossbreed dog still chews things like sheets, pillowslips, curtains etc. Never chewed any clothes, shoes or anything like that. She's a Border Collie cross - walks every day, on leash, for at least an hour - plays soccer, both at home and in a free leash area, been to obediance school, sits, stays, waits, comes, all of that on command. She just seems to have a thing about cotton type things - just can't resist chewing on them. Why?