This puzzle was set to explore anti-knight constraints. Although it's a nice little puzzle, there are other paths through it besides leveraging the full power of knight moves and it is only moderately difficult. It led to the much nicer Twelfth Knight (in the January 2022 archive). This puzzle also appears in the GAS Collection.

Estimated difficulty: 2/5

*Rules*

Standard sudoku rules apply. Digits within a cage cannot repeat and must sum to the total shown in the upper left corner of the cage (*standard killer cage constraint*). The same digit cannot appear in cells a chess knight's move apart (*standard anti-knight constraint*).

A straightforward puzzle with a clear solution path, Wind Up Wind Down has garnered many "great puzzle" and "beautiful logic" reactions from the community. It also appears in the GAS Collection and the Showcase Collection.

Estimated difficulty: 2/5

*Rules*

Normal sudoku rules apply. Digits in cages must sum to the number shown in the upper left corner of the cage (*standard killer cage constraint*). Additionally, each position in the 3x3 boxes contains a full set of the digits 1 to 9. In other words, no digit can show up in the same position in two different 3x3 boxes (*standard disjoint groups constraint*).

This puzzle has a rather esoteric rule set since it was built specifically to showcase a particular mathematical idea: complete sum-free sets modulo 10. Knowledge of that mathematics is not necessary to solve the puzzle but email us if you need a hint. This puzzle has received "great puzzle," "ingenious puzzle," and "mind-blowing puzzle" reactions in the CTC Discord channels. It also appears in the Heartburn Collection.

Estimated difficulty: 4/5

*Rules*

Normal sudoku rules apply. Digits on thermometers must increase from bulb to tip. Digits on purple lines are prime and can repeat along the line as long as they satisfy normal sudoku constraints.

Cages are sum-free modulo 10. That means that for any pair of numbers (including doubles) within the cage, the ones digit of their sum cannot be within the cage. For example, if 6 and 7 are in the cage, then neither 2, 3, nor 4 can be in the cage since 6 + 6 = 12, 6 + 7 = 13, and 7 + 7 = 14. Note that while the sum-free condition includes doubles, digits cannot repeat within a cage.

This puzzle came out of playing with cloning roles and coloring strategies. A relatively straightforward puzzle, it has received several "great puzzle" and "beautiful theme" reactions in the CTC Discord channels. It also appears in Indigestion Collection 1.

Estimated difficulty: 2.5/5

*Rules*

Normal sudoku rules apply but cloning has gone horribly wrong. Four different gene cages (indicated by A, C, G, or T in the upper left corner) have replicated across the grid with mutations. Each copy of the original gene contains only digits from the original but order, orientation, and cage size may vary. Every copy of each gene (and its mutations) is indicated in the grid. For example, if gene A is 1-2, then every place that 1 is orthogonally adjacent to 2 in the grid will be marked with a gene cage labeled A. Bonds, indicated by white Kropki dots, have formed where two different genes have adjacent consecutive digits. All consecutive pair bonds **between genes** are indicated but there may be unmarked consecutive pairs within or outside gene cages.

We set this puzzle because we just like prime numbers. It was the second puzzle we set together and at some point we will finish setting its partner, Killing Me Powerfully. This puzzle also appears in the Indigestion Collection.

Estimated difficulty: 3/5

*Rules*

Normal sudoku rules apply. Digits in cages must sum to the number shown in the upper left corner of the cages (*standard killer cage constraint*). Numbers outside the grid indicate the sum of the digits along the diagonal shown (*standard little killer constraint*). Digits may repeat along little killer diagonals as long as they follow standard sudoku rules. NOTE: One of the little killer clues is there for aesthetic purposes; it's still true but not necessary!

We set this puzzle as an introduction to Kropki dot logic for a novice solver. It also appears in the Brassica Collection.

Estimated difficulty: 1/5

*Rules*

Normal sudoku rules apply. Cells separated by a white dot differ by one (*standard Kropki difference dots*). Cells separated by a black dot are in a 2:1 ratio (*standard Kropki ratio dots*).

Snowdrops was the first puzzle we set together. It was created in response to the CTC Discord monthly puzzle prompt for December 2021: ice and cold. We are inordinately pleased that our first puzzle together received "mind-blowing puzzle" and "beautiful theme" reactions. It has two variants, shown below.

Estimated difficulty: 3.5/5

*Rules*

The sun is shining but a few flakes of snow are beginning to fall, represented in this puzzle by white circles. Is the temperature dropping?

Normal sudoku rules apply. Digits along an arrow must sum to the number indicated in the circle from which the arrow emerges (*standard arrow constraint*). Digits on thermometers strictly increase from bulb to tip but are not necessarily consecutive (*standard thermo constraint*). Cells separated by a white dot are consecutive (*standard Kropki difference constraint*). In addition, cells that are a chess king's move apart cannot contain the same digit (*standard anti-king constraint*).

We set this puzzle as one of two variants to Snowdrops. The solve has the same initial path but different snowflakes lead to a different solution. This puzzle also appears in the Indigestion Collection. Estimated difficulty: 3.5/5

*Rules*

The sun is still shining but the snow is falling faster. The snowdrops are reaching for the sky.

Normal sudoku rules apply. Digits along an arrow must sum to the number indicated in the circle from which the arrow emerges (*standard arrow constraint*). Digits on thermometers strictly increase from bulb to tip but are not necessarily consecutive (*standard thermo constraint*). Cells separated by a white dot are consecutive (*standard Kropki difference constraint*). In addition, cells that are a chess king's move apart cannot contain the same digit (*standard anti-king constraint*).

This is the second variant to Snowdrops. Again, a similar entry to the puzzle but a different constraint changes up the logic and solution a bit. This puzzle also appears in the Indigestion Collection. Estimated difficulty: 3.5/5

*Rules*

The sun is still shining but the rain is starting to fall. Will the rain make the snowdrops grow?

Normal sudoku rules apply. Digits along an arrow must sum to the number indicated in the circle from which the arrow emerges (*standard arrow constraint*). Digits on thermometers strictly increase from bulb to tip but are not necessarily consecutive (*standard thermo constraint*). Cells separated by a black dot are in a 2:1 ratio (*standard Kropki ratio constraint*). In addition, cells that are a chess king's move apart cannot contain the same digit (*standard anti-king constraint*).

This was a solo set by FullDeck and the rules are a bit long but it's a great puzzle if you're already familiar with sandwich sums. It was posted to the CTC Discord archive on 12/21/2021 and has received multiple "exceptional puzzle" and "beautiful logic" reactions from the CTC community. This puzzle also appears in the Heartburn Collection. Estimated difficulty: 4/5

*Rules*

Normal sudoku rules apply. Thermometers are strictly increasing from bulb to tip (*standard thermo constraint*). Numbers outside the grid indicate the sum of the digits between 1 and 9 in that row or column (*standard sandwich sum constraint*). Some sandwich sums are indicated by dashes and you must determine their value. However, each sandwich sum (whether indicated by a value or a dash) is a distinct prime number and those values increase from top to bottom and from left to right. Cells separated by a black dot are in a 3:1 ratio. Not all black dots in the grid are indicated. **NOTE:***The locations of the mystery sandwiches will not show up on the CTC app but they are in columns 4, 6, 8, and 9 and in rows 2, 5, 6, and 7.*

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