If a day is 24 hours long, does this mean night does not exist?
Pupils still struggle with this type of scientific conundrum years after they have tackled the topic in school, according to new research.
Academics at the University of California in Santa Barbara questioned 475 pupils about their understanding of the concept of a day.
The pupils, aged between six and 14, were asked to write or draw anything that illustrated their appreciation of the term.
The youngest pupils drew on their own lives for inspiration. One six-year- old wrote: "A day is when I go to school." Another volunteered: "A day is when the sun is up and you can play", accompanied by a picture of a boy playing outdoors.
By Year 4, pupils had learnt that the earth rotated around the sun, and they were therefore expected to demonstrate more sophisticated scientific understanding.
But 60 per cent were still preoccupied with the personal. Only a few were able to differentiate between a day and daylight. One wrote: "A night is a time when people are asleep, and day is a time when people are playing. Now put it together and it makes a day."
Their answers revealed significant confusion over the difference between the personal and the scientific. While they had absorbed lessons about the earth's cycle, they struggled to link these concepts to their own lives.
One pupil wrote: "A day is sometimes rainy and miserable or it could be sunny and joyful. Days always, always, always have 24 hours."
Making the same point in less florid terms, another pupil said: "A day is when it is light outside . A day is 24 hours long."
A third made the problem more explicit: "A day is 24 hours, but that would mean night doesn't exist."
The researchers said: "A well-educated adult will use the word `day' to mean the 24-hour period between two consecutive sunrises . and also to mean the roughly 12-hour period between sunrise and sunset characterised by daylight.
"Despite this complexity, we expect children to develop an understanding of day and night cycles as early as (Year 4)."
It was not until Year 6 that pupils' understanding began to change significantly. Almost three-quarters of these pupils used the words "24 hours" in their answer.
By Year 8, answers had changed again. This time, pupils referred to the earth's rotation. The most sophisticated discussed different time zones and the fact that countries facing the sun experience daylight.
In fact, the academics discovered, many concepts taught to pupils between Years 3 and 6 were not fully understood until Year 7.
They believe it is vital for primary teachers to recognise this delay. While it may be obvious when pupils do not understand specialised terms, such as "electromagnetic", their failure to grasp the meaning of everyday words, such as "day", often go unnoticed.
"In science classrooms, students and teachers may misunderstand one another, not because they necessarily disagree on science concepts, but because they are using different (and sometimes tacit) definitions of words," they said.
"If a few variations are identified, teachers can better attend to the needs of all students."