It always bugged me that the liar participates in explaining the rules. That, and they never explaing that saying "It's a piece of cake" is the reason she falls down to those creepy grabby hands.

As a kid, I always assumed she went through the wrong door despite solving the riddle because the rules of the game were explained by a liar, and that they were both liars.

Since everyone in the comments seems to know, what's this a reference to?

Take chicken to other side. Return and pick up the fox. Take the chicken back to the starting point and pick up the grain. Leave grain with fox and return for chicken.

Since you can only ever leave the grain and fox together, your hand is forced at every step. The order of the grain and fox is arbitrary.

If one of you always lies, can't I just assume the one to explain the premise is the one that tells the truth?

The fox, the grain, and then the chicken

Edit: no wait, you're still leaving the grain alone with the chicken. So you take the chicken across, then go back for the grain. You drop the grain off, and take the chicken back with you, then drop the chicken off and take the fox and take it across, then go back for the chicken

"You ever fuck that guys wife?"

Be careful, he lies!

Now try this one.