Britain's children are still struggling to spell simple words like "doesn't" and "believe" but have little problem with "pterodactyl" and "archaeologist", research suggests.
An ongoing study by the Oxford University Press reveals that simple words are proving to be a challenge for the nation's youngsters.
It found that when children use less common words in their writing, they are likely to spell them correctly almost all the time. But everyday words are often incorrectly spelt.
Experts said the mistakes are likely to occur because children will look up words they rarely use, but do not do the same for those that they come across on a regular basis.
Researchers analysed more than 33 million words written by children aged seven to 13, looking for the types of words that were correctly and incorrectly spelt.
The findings showed that youngsters were able to spell words such as "palaeontologist", "pterodactyl", "archaeologist", "cerulean", "psychologist" and "brachiosaurus".
The words children had the most problems with included "doesn't" which was written as "dosen't" or "dosent", "surprise" which was commonly spelt "suprise", and "until" which was spelt "untill" or "intil".
Lexicographer and English teacher Jane Bradbury, who is part of the research project, said: "The majority of errors are occurring in commonly used words with spellings that are difficult to guess and, in English, there are a lot of words with unguessable spellings!"
Many spellings are difficult to guess because they contain silent letters or syllables which are not heard when the word in spoken, she said.
Vineeta Gupta, the OUP's head of children's dictionaries, said: "Children are keen and motivated to spell well, and it is pleasing to know that they probably look words up that are technical or more complex. At the same time, children are still struggling with simple and everyday words."