"We're going to be buying our Christmas tree this weekend, and we'd like to make a fresh cut on the trunk to help it with water uptake. What kind of saw do we use? At the lost we used to buy from they'd use a chainsaw, but we don't want to buy a chainsaw to use just once a year. So what kind of saw do we need? Is a hacksaw going to be strong enough?"
Don't use a hacksaw to cut down a tree. It's got shallow teeth and it'll take forever. Get a bow saw or a regular hand saw that has more aggressive teeth and it'll work fine. Most people prefer a bow saw for tree cutting because the thinner blade is less likely to get trapped as you cut through the tree and it settles on the blade, but unless the tree is more than ten feet tall I never saw (har har) much difference.
"I've always wanted to know this - how do container ships keep the shipping containers from sliding off the deck during rough seas? I know the containers themselves are really heavy and big (they only look small compared to the rest of the ship!) but I would think one rogue wave and all those containers would be at the bottom of the ocean."
They clamp to the deck and each other, so barring huge force or a failed clamp they won't tip. Of course, failed clamps happen more often than you'd think so rough seas can cause tipping and spilling but most container damage is due to loading accidents.