Click the green flag to start. This ScratchCat will ask you what number of the Fibonacci series you want. The Fibonacci series are a set of number where the rule is: Add the current number to the previous number to get the next number, i.e. 0+1=1, 1+1=2, 1+2=3, 2+3=5, 3+5=8, 8+5=13, etc. So the pattern is: 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, etc. Don't add extremely big numbers or negative numbers, it will lag the cat out.
I suggest putting the project in TURBO MODE for big numbers. In Turbo Mode, 10,000 takes about a second to calculate.