| Apparently. It seems that they flow r e a l l y s l o o w l y. The reason they can still hold your shopping is that the polymer molecules (polyethylene - or rather LDPE) are a sort of tree shape that forms stronger network interactions than unbranched chains. Like many substances formed from chains (jelly/jello, dough, hair) alignment of the chains makes the material stretchy while tangling makes it stiff. Heating the plastic bag a little (too much will burn it - you KNOW that's bad!) will melt it and it will flow. Think cooked spaghetti (mmmm..spaghetti!).
LDPE, on the other hand, is so entangled with inter-chain contacts that flow is impossible. However, once the force on a bag exceeds a certain limit, it deforms plastically - that is it goes all stretchy and dumps your eggs and milk on the floor. Hungry now :-( | 
