This designer diary was originally written for the Tiny Library kickstarter.

The concept for Itsy Bitsy Spider came to me in a flurry of inspiration. I'm very proud of how concise and thematically tight this game is. I really like how the payoff of the game reflects the perseverance of the spider trying and trying to finally reach the top of that damn spout. My partner pancelor helped create a program and run some numbers to see just how hard the game is, because I wanted it to be challenging, but not impossible. I was playtesting it ‘by hand’, but that really isn't very fast! Simulating hundreds of games at once with a computer (using PICO-8 to be specific) made that a lot easier. 

The rules of the game are as follows:  You start with a height of 1. Roll a d6: if you roll is greater than or equal to your height, gain 1 height. If your roll is less than your height, lose 2 height. When your height reaches 6, you win!

Here's a graph that shows the spread of games based on the number of rolls to win. On the x axis is the total number of rolls needed to reach a total height of 6. The y axis is the number of games played. Each bar in this graph is a range of 8; so the games in the first bar took 0-7 rolls to win, the second bar took 8-15 rolls to win, and so on. The taller the bar, the greater the number of games. 

To get a sense of the difficulty of Itsy Bitsy Spider, I reran this simulation multiple times to see here the majority of the games ended up. In the above gif, each frame represents a simulation of 256 games. As you can see, in each instance the bulk of the games falls somewhere between the 0-32 rolls, with the majority under 64 rolls. The distribution does have that long tail though, so there are still the few games that could take a lot of rolls. If luck isn't on your side, you could be climbing that spout for a while. 

I played around a bit with the numbers (how much you gain or lose, what your final height should be, etc) before I settled on the final game’s rules. I chose this because I like the spread of this graph the most. It gives a nice mix of sometimes you get lucky and get to the top quite easily, sometimes you might do a few 10s of rolls before you win, and then there’s the occasional game that lasts a long time as you desperately claw your way to the top.

If we adjust the rules to have you lose one height instead of two, you can see that the games are skewed a lot more to the left; i.e. in more of the games it’s going to be a lot easier to win and it will take a lot less rolls to get to the top. That second bar is so tall in almost every instance - the majority of games would be won in 9-15 rolls. 

In the end I decided not to go with this ruleset. Determination and perseverance are what gets the itsy bitsy spider to the top, and the frustration and challenge of the journey make the pay off of overcoming the rain all the more rewarding.