You might be familiar with the concept of Schrodinger's Cat; it's a scientific experiment which demonstrates that under certain test conditions, if a cat is sealed into a box and nobody can see it, it can be perceived as alive and dead at the same time. In a similar way, your child's stomach is both simultaneously full and empty every minute of every day.
Your child will stubbornly insist that they couldn't eat another morsel, and then expect to be fed if something more interesting than their last course arrives on the table. You'll ask them if they need to eat anything before you set off away from the house and they'll say no, and then they'll be hungry five minutes later in the car. Unfortunately, the same phenomenon also seems to affect their bladder. Always carry car snacks and know where toilets are!