At least most of the tough nuts in Shivers are very old classic puzzles that you can look up in puzzle books. Which is what I ended up doing with the marble solitaire puzzle - this is purely mathematical, and it's something you can either do or you can't.
The 7th Guest and The 11th Hour (in my opinion) aren't really adventures so much as puzzle collections with a storyline running through them. The 11th Hour makes it clearer that at the end of each "level" is a game played against Stauf, not just an ordinary puzzle - sort of the equivalent of a level boss.
Yes, the bacteria game in The 7th Guest is a nasty one. If anyone's still stuck on it, here are a few tips. Play aggressively, REALLY aggressively. You can learn some strategy by seeing how Stauf plays - if you can predict what move he'll make every turn, you're learning. And finally, Stauf
usually plays with computer-perfect accuracy, always making the best possible move each turn. But every now and then, very very rarely, he screws up. This is your big break, as long as you can spot it and take advantage of the opportunity.
Two frustratingly illogical puzzles that stick out in my memory come from the very first Zork. One room is terribly noisy and echo-ey, so you can't stand to stay in it long enough to pick up the gold bar (or was it a platinum bar?) in the middle of the floor.
Spoiler:What you have to do is say "echo", and that changes the acoustics of the room so you can stay in it. I tried saying a lot of different things in there, but never "echo". It's just not something you say.
Even worse than that was the rainbow puzzle. It's fairly obvious that you need to find some way to use a rainbow as a bridge across the gorge.
Spoiler:It turns out that what you have to do is take the sceptre from your treasure collection to the foot of the rainbow, then wave it like a magic wand, thus turning the rainbow solid. Now, who thinks of waving a sceptre? If the thing had been described as a wand, I would have thought of it.
