TFA will inform you of this though, it's intended to be a game of reasoning. Posting the answer is spoiling the fun for people who read the comments first
This is a very common example in math classes, though in my experience usually presented as a proof that "all horses are the same color".
No cat has eight tails, and every cat has one more tall than no cat. Ergo...
> See if you can figure out in which step the fallacy lies. When you think you've figured it out, click on that step and the computer will tell you whether you are correct or not, and will give an additional explanation of why that step is or isn't valid.