A space ship with it's three astronauts went astray in space and reached an alien planet. There were big monster like creatures there and they took captive of our three men. They were also highly intelligent. Their technology was also highly advanced. They were able to decipher our English language.
   Our men talked to them and requested them to release them. So that in future our two planets may remain friends. But the aliens told that they can be friends with us only if we humans are equally intelligent to them. So they wanted to test the intelligence of our three men.
They brought 3 numbers of apple like fruit which they called as popo and 2 bitter gourd like vegetable which they called as jito. So totally there were 5 of them.
They showed them to our men. Then our three men were made to stand in a straight line in such a way that person 1 was on the front and can't see the other two. Person 2 was standing behind person 1, and can see person 1 alone. Person 3 was made to stand behind both of them and can see person 1 and 2.
Now they were blindfolded. The aliens placed the popo and jito one on each person's head. So, totally three of them were placed one on each person's head. Then they hid the remaining 2.
  Now, their eyes were opened and they were given the condition if at least one person can correctly tell whether popo or jito is placed on their own head, then they will free them and will remain friends with our earth.
The person 3 said that he cannot say what is placed on his head on seeing the other 2 person's heads. The person 2 also said that he cannot say what is placed on his head after seeing what is placed on the head of person 1. On hearing this, Person 1 said that he can tell what is placed on his head. What was placed on person 1 head? How did he find that?

Friends, this is really very very tough.. maybe the toughest one  of my riddles... 
Let's see who can crack this?