Stiffness, elasticity and pressure. BUGGY BUILDING BLOCK SERIES DOLL SERIES R/C CAR SERIES R/C PLANE SERIES FURNITURE APPLIANCES SERIES FREE WHEEL SERIES PARKING LOT CANDY TOY B/O SERIES DIE CAST SERIES TRACK SERIES SPORT SERIES DOCTOR SERIES TOOL SERIES GAME SERIES BEAUTY SERIES KITCHEN SERIES FRICTION SERIES PULL BACK SERIES PULL LINE SERIES WIND UP SERIES WATER GUN SERIES GUN SERIES LEARNING SERIES CREATIVE PLASTICINE SERIES VINYL SERIES BEACH SERIES EDUCATIONAL SERIES BABY SERIES MUSICAL INSTRUMENT SERIES ANIMAL SERIES FISHING SERIES INFLATED SERIES BUBBLE SERIES DEFORMATION SERIES MILITARY SERIES DECORATION SERIES MAKE UP SERIES FESTIVAL,PARTY,FUNNY SERIES YOYO,TOP SERIES DIY SERIES