Classic Mind-benders

You have 8 identical-looking balls. One of them is slightly heavier than the rest. Using a balance scale (no weights, just tells you equal or which side is heavier), Aadi thinks finding the heavier ball needs at least 3 weighings. His sister says it can be done in just 2. Who is right?

A farmer must cross a river with a fox, a chicken, and a bag of grain. The boat can carry only the farmer and one item at a time. Leaving the fox alone with the chicken is dangerous (the fox eats the chicken), and leaving the chicken alone with the grain is also dangerous. Yash thinks this crossing is impossible. His teacher says it is possible with the right order. Who is right?

You have two ropes, each taking exactly 1 hour to burn completely — but each rope burns unevenly (half the rope might not take half an hour). Using only these two ropes and matches, Navya thinks measuring 45 minutes is impossible because the ropes burn unevenly. Her friend says there is a way. Who is right?

Four people want to cross a bridge at night with just one torch (the torch is needed to cross). Crossing takes them 1, 2, 5, and 10 minutes respectively; crossing together means going at the slower person's pace. Rohan thinks always taking the fastest person (1 minute) along as an escort is the best method, taking 19 minutes total. His sister says there is a better way — in 17 minutes. Who is right?