throw

In order to throw an exception from your own code, you say throw, new, then the name of the exception and ().

RuntimeException is one of many kinds of exceptions, but you can get by with just that one for a bit.

void crashesOnFive(int x) {
    if (x == 5) {
        throw new RuntimeException();
    }
}

void main() {
    crashesOnFive(1);
    IO.println("Made it to step 1");

    crashesOnFive(5);
    IO.println("Will not make it to step 2");
}