If you don't know

In Spain there is a river called the Tajo that runs through the city of Toledo. There is also another one called the Tejo. A poet decided to share a poem about the river Tejo, and at a festival he recited it:

-A orillas del río Tejo, la prenda de mis amores... (On the banks of the river Tejo, the darling of my heart...)

A Galician interrupts him and says:

-Hey, río Tajo, río Tajo!!

The poet goes back to reciting his poem:

-A orillas del río Tejo, la prenda de mis amores.…

-Hey, it’s called río Tajo, he interrupts again.…

Once more the poet begins:

-A orillas del río Tajo, la prenda de mis amores...

Se miraba en sus reflajos como si fuera un espajo (She gazed at herself in its “reflajos” as if it were an “espajo”: reflejos and espejo, reflections and mirror, mangled to rhyme with Tajo)

¡¡Para que veaís carajo, que era Tejo y no Tajo!! (So you can see, damn it, that it was Tejo and not Tajo!!)

Taken from http://www.escueladelarisa.com.ar/cuentos.htm

How many times have you run into people who behave just like the Galician in the story?

They hold forth without knowing, without any detailed idea of what is being discussed, without any grasp of the context. Based on what they know about two or three loose points they opine as if they had mastered the whole subject.

Tell those people this joke about the poem of the river Tejo, and while you’re at it tell them that if they don’t know what the poem is about, they’d better keep nice and quiet.