Iván C. Filpo wrote:
Thanks for responding with such a nice website, I have just added it to
my Google Notes for further reference. However, I do not think the
problem has to do with worn sprockets because it is a brand new bike
and everything looks not worn out. I was thinking that it had to do
with the little "lining sproket" that lines the chain with the sproket
that would need realignment, but does not always happen.

Since it's a Mongoose, I'm going to guess that you bought it at a
department store. So you're probably not going to get help from the
retailer after the sale; in other words, you're on your own.

I suspect derailer maladjustment based on your limited description of
the problem. Someone has pointed you to Sheldon Brown's page. Go there
and find his article on derailer adjustment. And good luck. I hope the
shortcomings of a department store bike doesn't sour your cycling
experience.

--
Dave
dvt at psu dot edu

Everyone confesses that exertion which brings out all the powers of body
and mind is the best thing for us; but most people do all they can to
get rid of it, and as a general rule nobody does much more than
circumstances drive them to do. -Harriet Beecher Stowe, abolitionist and
novelist (1811-1896)