da far, what is the simplest invention, Phineas and Ferb have made( That can be done da a real living Teenager)?

i suppose the simplest is popularizing the word 'AGLET' which could be done with a little money in hand!

I guess the only one I can think of off the superiore, in alto of my head is when they become pop stars, because all they had to do was write a song and put it up probably for free download. Now I'm not sure if they could have become that popolare in one day, but they didn't need any money really, just musical talent, which I'm pretty sure they have.

If I think, Perry the Platypus action figure is quiet simple too but only some living teenager could do that but there's one thing are very simple is a birthday card that Phineas and Ferb made for their mum and even kids could do it also we don't need much money to spend with!

is that the one water slide in the episode "do nothing day"?

None I don't think this is possible for one kid to invent something. Maybe when they get older they'll do it.

They "invented" a cuore of rock and roll in Baljeet, that's pretty easy to do.

oh defiantly the titolo sequence

"mom phineas and ferb are making a titolo sequence"
hehehehehe

the limonata stand(excluding the giant robot making the lemonade)