Click/tap the cat and enter a hexadecimal (base 16), octal (base 8), or binary (base 2) number (without any prefixes), say which one it is, and watch as the decimal equivalent comes out.
This uses a convenient feature in JavaScript (the language Scratch 3.0 is programmed in, versions ES6 and above) that allows you to prefix a number with "0x" to make it be considered as hexadecimal, "0o" for octal, and 0b for binary, and Scratch converting the string output of a join block into a number when it's in an addition block input. Scratch also always outputs base 10 from mathematical operators- so you can just add (or subtract) zero from the number you input to get the base 10 equivalent. Unfortunately this doesn't work both ways.
