Jim and his mum and dad went out in the rain on their holiday but only two of them got their hair wet why?

Jim and his mum likely used umbrellas or wore raincoats that protected their hair from getting wet, while Jim's dad may not have had any protection from the rain. Alternatively, it could be that Jim and his mum stayed under cover while Jim's dad ventured out into the open rain. Consequently, only two of them ended up with dry hair.