How do you investigate, disassemble and evaluate a product which has mechanical and electrical components? Search me guv. How do micro-organisms break down waste, or cause diseases? Er, let me think nowIHow do the valleys, rivers, multi-storey car parks, movement of goods and people in my town compare with a similar locality in Africa, Asia (excluding Japan), South or Central America? Just a second, it’s on the tip of my tongue.

There is no need to worry about the answers to these searching questions. They all concern topics that the law says primary teachers must teach in just three of the national curriculum subjects they have to cover at key stage 2, namely design and technology (knowledge and understanding - “products and applications”), science (life processes and living things - “living things in their environment”) and geography (“places”).

I know lots of primary teachers, so my problems are over. I can just ask any of them.

When the national curriculum was simplified, the assumption was that the ferocious demands on primary teachers’ subject knowledge had been hugely reduced. The volume of words describing each subject had certainly been thinned out, and some of the content was slimmed, but much of it never went away.

Take as an example electricity and magnetism. Back in 1989 they had an attainment target all to themselves. These two dear friends were the whole of attainment target 11 in science, when Kenneth Baker wanted every child to be tested on every one of 60 or 70 different ATs.

In May 1991 it was proposed that there should only be five ATs and our twin chums electricity and magnetism were swallowed by number five, “forces”. That lasted until September 1991, when they officially became part of a new number four, “physical processes”.

After Ron Dearing’s slim-down, “magnetism” disappeared from key stage 1, though “electricity” survived. Both are still alive and well at key stage 2, so demands on teachers’ subject knowledge have not abated by much. Children still ask, as they did in 1989, “Please Miss, what happens inside a magnet?”, “Please Miss, what is a battery made of?”, “Please Miss, if you stuck 500 batteries together and wired them up to someone’s big toe, would he light up?” The vast stores of knowledge primary teachers are supposed to possess makes them fine candidates for Brain of Britain or Mastermind. In both programmes you can bone up on the knowledge, but it is the questions that are the killers. Even searching books and databases for answers is not easy if you do not know which sub-heading to look for.

The complicated topic “floating and sinking” is a good example of this dilemma. Primary classrooms were once awash with children dropping pineapples, weights and oranges, with and without their skin, into bowls of water, while their teachers prowled round making notes on soaking wet clipboards. The topic may not now hold the position it used to occupy, but it is still there. Under the heading “balanced and unbalanced forces”, for example, children can still float objects in water to see what forces are acting on them.

It is not difficult to prepare activities, but when children’s questions start to fly, then teachers’ own subject knowledge can creak. Why do we float more easily in the sea than in fresh water? Simple. Salt water is 3 per cent more dense than fresh water. How much lighter are we in water anyway? Easy peasy. As Archimedes discovered more than 2,000 years ago, we become lighter by an amount equal to the weight of the water our body has displaced. All right then, clever dick, how do some insects manage to walk on water? What a facile question. It’s all because of surface tension, silly. The inward attraction of molecules of liquid on the surface of the water to other liquid molecules, has the effect of forming a sort of skin over the water.

But suppose you don’t know that “density”, “Archimedes’ Principle”, and “surface tension” are what you need to look up? And how can you best explain these ideas to curious primary pupils in terms they understand? It is not just a matter of subject knowledge, but of choosing suitable teaching strategies.

It is not surprising that most teachers have developed nifty footwork. One common ploy is Steinberg’s Reflective Strategy, named after Sid Steinberg who turns every unanswerable question back on the class. “That’s a very interesting question, Damian, so for homework tonight, Year 5, I want to see if you can find out why wagtails wag their tails.” This tactic has the advantage of sounding pedagogically smart, as you can always, if asked, mumble something about “ensuring that children learn to think for themselves in the challenging world of the 21st century”.

Another strategy is the Pickersgill Backhand Lob, perfected by Mavis Pickersgill, which dispatches thorny questions deep into the back of the court with, “Thank you for that important question, Gemma. Now, whose turn is it to look up the answer in that shelf full of 1,000-page encyclopedias at the back of the room?”.

Then there is the story of the Nobel prizewinner, who said to his regular chauffeur, “You must have heard my standard lecture so many times, I bet you could deliver it and nobody would know you’re not a Nobel prizewinner.” The chauffeur duly gave the lecture, word perfect, but to his horror someone stood up at the end and asked a mind-blowing question. He must have been an ex-primary teacher, as he answered, quick as a flash, “That question is so simple even my chauffeur could answer it.” If all else fails, keep a Nobel prizewinner at the back of the room disguised as a chauffeur.

Ted Wragg is professor of education at Exeter University