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.
TV presenter Kate Garraway, a supporter of Read On. Get On, said she was fascinated by her two children's curiosity.
She said: "Recently my six-year-old asked when we were out in a cafe 'why do girls toilets have a picture of a person with a skirt on? Girls wear trousers all the time and in Scotland boys wear skirts.'
"That one I did know the answer to! But often I don't, so it's fun to look up the answers together on the internet which always gets a conversation going."
DJ and mother-of-two Lauren Laverne, who is also backing the campaign, said her children's philosophical questions were "really above her pay grade".
She added: "My youngest is very imaginative. He always asks 'are we alive or is this a dream?'".
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, Holly and Topsy and Tim in everyday fun situations and are available free from the Read On. Get On. website.