After reading this question and its answer I realized. How come one of the twins, Fred or George, does not matter that much was able to transform Ron's teddy bear into a great spider? Let's quote Professor McGonagall:
‘Transfiguration is some of the most complex and dangerous magic you will learn at Hogwarts,’ she said. ‘Anyone messing around in my class will leave and not come back. You have been warned.’ Then she changed her desk into a pig and back again.
After this, everybody is excited to transfigure something. But:
After making a lot of complicated notes, they were each given a match and started trying to turn it into a needle. By the end of the lesson, only Hermione Granger had made any difference to her match; Professor McGonagall showed the class how it had gone all silver and pointy and gave Hermione a rare smile.
Harry Potter and the Philosopher's stone, ch. 8, The Potion Master
And this is only one of the mentions about Transfiguration being one of the hardest magic. Not to mention, that wizards normally get a wand at the age of eleven. Although, against this argument might stand the fact, that Weasley family was rather poor, unable to buy new wand for Ron - he used old Charlie's wand, so Fred or George might have nicked the wand.
I mean, Fred or George would not care, if it was dangerous - on the contrary. But how come, was one of them able to do such magic at the age of five? Think about what he would need:
- a wand (I assume from what J.K.Rowling said about wandless magic)
- to know the proper incantation
- the proper pronunciation/articulation of the incantation
- the correct wand moves