What I notices and learn about Number Fact Families!

What I've notice about Number Fact Families,  the mos thing I notice about this website is that  it gets harder once you finished the the 
level and that it's looking for the right one like if you just do any random one and it's right they will mark it as wrong. 
Thing's I've learnt the most things that i have learnt is that you need the real number fact family's  and now I know new Number Fact Family's.