I have to disagree with Wayneburg's observations of milk jugs. While they are generally pretty tough, I've had them rupture when they hit the ground after the grocery sack ripped open. This was factory sealed, & full of milk, & required a trip back into the store for a replacement. I've also dropped one onto the floor & had it rupture.

I've also seen 2 & 3 liter soft drink bottles rupture when dropped (shatter is more like it). They're especially vulnerable when the cap hits the ground. I still have Pepsi spots on the interior ceiling of my Suburban from one dropping to the pavement when the grocery sack ripped open (Hmmm...maybe I'd better either sack my own groceries, or change grocers...) This has happened more than once.

Extreme temperature changes for a long term, and especially exposure to sunlight (UV rays?) will weaken milk jugs & soft drink bottles.

It seems as if they're tough as nails when I'm trying to destroy one, and fragile as glass when I'm not expecting it to break. YMMV.

David