Does your script look something like this?
Now we need to calculate what the new value is, and put it in the variable called "new". In Scratch, you can put a variable anywhere there is a white box where you can type. Look under the green operators tab. You will see different mathematical operations you can do on numbers you type in, or on variables. Which operator do we need for the Fibonacci sequence?
Your task: When the spacebar is pressed (hint: look under control, make the cat set the variable "new" to the next number in the sequence. Then, set second-to-last to be equal to last, and last to be equal to the old "new" value, so that they will be ready for the next time you hit space. If you want to make the cat say the answer rather than just having it appear in the variable box, try looking under the looks tab.