A “brain-teaser” is something that makes you use your brain. You need to be a logical thinker to get the answer to this!
You are on the bank of a river. You have to get a fox, a hen, and corn to the other site of the river. If left alone, the fox will eat the hen, the hen will also eat the corn if left alone. The boat is only big enough to take you and one of the other three to the opposite side. How do you get all three across intact?
Answer: First take the hen across. Leave the hen. Go back and get the fox. Take the fox to the other side. Leave the fox there, but take the hen with you back to get the corn. Leave the hen and take the corn to the other side. Drop the corn off with the fox, then go back to get the hen. Bring the hen to the other side. All three make it fully intact!