Haha, been there — my nephew once asked me the same and I totally froze. Easiest way to explain it: arteries carry blood away from the heart under higher pressure, so their walls are thicker. Veins bring blood back and have little valves to keep it from flowing backward. Think highways vs slower return roads. When we were prepping his poster, this page helped me not butcher the facts: how are arteries and veins different . He even used the traffic analogy in class and apparently the teacher loved it, so your dad credibility should be safe!