It sounds like an April Fool’s Day joke — but it’s not! Amazon’s just released a brand-new free game for the Kindle. It’s a beautiful rendition of the classic “number slider” puzzle — this time with a couple of twists.

To start with, there’s just one empty square on a grid of numbered titles, which makes it possible to slide just one tile at a time, either up, down, or sideways. “Your goal is to use the empty space to slide the numbered tiles until they are in order,” Amazon’s instructions explain. “When the tiles are in order with the empty space in the bottom right, you win!”

But Amazon also lets you select a new difficulty level for the puzzle, offering grids that are either “small, medium, or large.” (That is, you can slide the numbers in a small three-by-three grid, a trickier four-by-four grid, or an even more challenging five-by-five grid.) And if you choose the “automatic” setting, a tile will move as soon as you highlight it, so you don’t even have to press in the select button. (And if you instead you choose the “manual” setting, you can cursor past several tiles, and then move them all at once by selecting the one that’s farthest away!)

“You know, the last thing I need is yet another addictive game on my Kindle!” complained blogger Michael P. Gallagher. This morning he posted the game’s first review on its page in the Kindle store, writing that “The graphics are very crisp and the response time is very fast on my Kindle 3 as compared to the slowness I saw on the free Kindle poker game…”

“Now, if I can just find time to read on my darn Kindle….”

This game is part of an unacknowledged trend, since gradually all of the classic games are starting to become available on the Kindle. Just yesterday Oak Systems Leisure Software released Codewords and Cryptograms for Kindle. It’s the familiar cryptograms that appear in your daily newspaper, where a quote from a famous person is hidden with a “substitution” code where different letters are swapped in to represent every letter. (And they’ve also bundled in a fascinating variation on the classic crossword puzzles, called “Codewords,” where you try to perform the same de-ciphering in a crossword puzzle grid!)

That company also released the first Kindle version of Chess in February (as well as a Word Search game). And if you’re looking for traditional crossword puzzles, The New York Times has released six different volumes. One week ago, two different companies even released two different Kindle versions of the board game checkers on the exact same day. And just Tuesday, the same thing happened again, when two companies released competing versions of the disk-flipping game Reversi.

In February, a company named 7 Dragons released a Kindle version of the game Tic Tac Toe (as well as a new game called Flip It) — but two weeks ago, they even unveiled a Kindle version of the classic text-document application, “Notepad.” And there were already two competing versions of the software. So game development is definitely starting to happen on the Kindle platform.

I agree with Michael Gallagher — all these games are cutting into the time that I’d normally spend reading on my Kindle. Number Slide marks the sixth free game that Amazon has released. Below is a complete list of all Amazon’s free Kindle games — in case you’re looking for more fun ways to spend this year’s April Fool’s Day.