[Note: I received a free copy of these puzzles in exchange for a fair, unbiased review.]

Forgive these somewhat shorter reviews, but it’s really hard to talk about brainteasers and puzzle boxes without accidentally spoiling them for users. So you get a two-for-one review today!

The perfect puzzle gift box needs to walk a difficult tightrope. It has to be challenging enough to justify the cost, intuitive enough that a person understands the basic idea without instructions, and yet not difficult enough to ruin the holiday or event by causing frustration.

(Cue a vivid Christmas memory of my sister shaking a puzzle box so hard she accidentally solved it and sent a gift card flying across the room.)

The Roman Lock Box walks that tightrope deftly. It manages to combine classic puzzle box elements (symbol clues, moving elements requiring careful positioning, parts that look seamless but aren’t) and combines them nicely with a sliding-block puzzle that offers a far different challenge than the usual puzzle box tricks.

Factor in the clean design, the well-made pieces, and the delightful slot in the side (allowing non-solvers to place their gift inside without actually having to solve the puzzle themselves!), and you have a suitably challenging way to keep your giftee busy for a bit.

[The Roman Lock Box is for ages 8 and up, and it’s available from Project Genius and participating websites, starting at $17.99.]

Our second brain teaser is a total 180 from the previous one. The Roman Lock Box has a number of moving parts and things to fiddle with and try out, offering a plethora of options to a solver.

Archimedes’ Gear, on the other hand, is mind-scramblingly rigid. Brutally challenging in its simplicity. Twist and turn it, flip it, rearrange it, hold it at all angles, and it seems to give you nothing. I don’t know that I’ve ever been left so baffled at the start of a solve like I was with Archimedes’ Gear.

Naturally, that’s part of its genius. It really feels like a piece of ancient technology that fell into your hands, and you’re missing some valuable bit of insight that would help you operate it.

That initial bafflement does give you ample time to admire this absolutely beautiful puzzle, though. The color choices, the materials, the feel of the puzzle as you examine it from all sides. There’s even a lovely auditory quality to it, hinting at what awaits the solver inside if its secrets are uncovered.

This puzzle is ranked a 4 out of 5 in difficulty, and I think that’s very fair. I’ve solved a number of brain teasers and puzzles from Project Genius, and Archimedes’ Gear felt a step beyond most of them.

Of course, that only makes it more satisfying when you finally realize what’s going on here. Not a puzzle for the faint of heart (or those lacking in patience and determination), but still one definitely worth your time.

[Archimedes’ Gear is for ages 14 and up, and it’s available from Project Genius and participating websites, starting at $29.99.]