"what would your brother say the way to the oasis is ?"
And you take the other way.
If you had asked the one who lies he would know his brother tell the right way but he has to lie, hence indicates the wrong way.
If you asked the brother who tells the truth, he knows the his brother would lie and indicate the wrong way. As he cannot lie he would indicate his brother's answer and show the worng way.
Knowing the wrong way, you know the right one !!
It goes all back to primary boolean logic :
FALSE x TRUE = TRUE x FALSE = FALSE
catch the drift ?
'til next time