Children ask "why?" eight times a day but parents are unable to respond more than half the time, a survey found.
Almost half (47%) of parents with children aged three to 10 noticed an increase in the number of questions during journeys, according to research by Read On. Get On, an organisation promoting literacy.
Parents find "how long will it take?" and "what happens when you die?" the most challenging questions to answer, followed closely by "where do babies come from?", often prompted by the birth of a sibling.
Science also presents a challenge. A quarter of parents admitted they were flummoxed by questions from budding Einsteins, including "why is the sky blue?" and "how many stars are there in the sky?".
But while nearly half (48%) of parents took the time to look up answers to their child's queries with them, one quarter (26%) admitted to being creative with the truth.
Speech and language therapist Kate Freeman said "why?" questions were crucial to a child's development.
She said: "Studies find that if children don't have strong language skills at age five they can get left behind when they start school and struggle with learning to read.
"That's why it's so important for adults to chat with children to help them develop the essential language skills needed to be ready to read when they enter the classroom. "
Read On. Get On. - in partnership with Ladybird - is launching a free nationwide giveaway of "story starters" - activities aimed at helping families chat with their children to develop their language skills.
They feature popular children's characters Peppa Pig, Ben & Holly and Topsy & Tim in everyday fun situations and are available free from the Read On. Get On. website.