Elizabeth, 11, and Teddy, 8, have never gone to school.
Their teachers are primarily their parents, which places them in what is believed to be one of the fastest growing sectors of the US education system -- the home-school movement.
For their science lesson, Teddy and Elizabeth are joined by three other home-schooled children, who live down the street in their suburb midway between Baltimore and Washington D.C.
Before the lesson starts, all five children change into Renaissance costumes -- long dresses and bonnets for the girls, tunics and swords for the boys.
''We definitely have a lot more fun than kids who go to school,'' Elizabeth said.
Nobody is quite sure exactly how many American children are being taught at home. The National Center for Education Statistics, in a 2003 survey, put the number that year at 1.1 million. The Home School Legal Defence Association, which represents some 80,000 member families, says the figure now is quite a bit higher -- between 1.7 and 2.1 million.
But there is no disagreement about the explosive growth of the movement -- 29 per cent from 1999 to 2003 according to the NCES study, or 7 to 15 per cent a year according to HSLDA.
This growth has spawned an estimated 750 million dollars a year market supplying parents with teaching aids and lesson plans to fit every religious and political philosophy. Home-schooled children regularly show up in the finals of national spelling competitions, generating publicity for the movement.
Parents cite many reasons for deciding to opt out of formal education and teach their children at home. In the NCES study, 31 per cent said they were concerned about drugs, safety or negative peer pressure in schools; 30 per cent wanted to provide religious or moral instruction while 16 per cent said they were dissatisfied with academic standards in their local schools.
''I wasn't sold on the idea of institutionalised education.
It's a factory approach -- one size fits all,'' said Isabel Lyman, author of ''The Homeschooling Revolution'' who taught both of her now-grown sons at home.
''The schools take all the joy out of learning. They don't take account of a particular child's interests, needs and development. The whole system is anti-child,'' she said.
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