* Draw two lines of equal length, one above the other. Draw arrowheads pointing outwards at both ends of one line, and arrowheads pointing inwards on the other. Add the label: "Which is the longer line?"
* Draw two circles of equal diameter and colour them red. Draw six large circles in a contrasting colour around the first circle, and six smaller ones around the second circle. Label with the question "Which red circle is the largest?"
* Draw two large equally sized circles side by side on a piece of paper.
Paint one in a dark colour and the other in a light colour. Label with the question: "Which circle is the largest?"
* Draw two equally sized rectangles side by side on a piece of paper and colour them in. Take a ruler and place it at a shallow angle across the first rectangle. Starting beyond the left side of the rectangle, draw a line along the ruler that stops when it meets the rectangle; do not draw over the rectangle. Starting at the right edge of the rectangle, draw a second line away from the rectangle to the right. Remove the ruler and the line will appear to pass behind the rectangle.
Place the ruler in a similar way across the second rectangle. Draw in the top line, but then move the ruler 2cm to the right, before drawing in the second half of the line. Label with the question: "Which line passes straight behind the rectangle?"
Mount these optical illusions on the wall and ask your students to write down their answers. Then get them to test their answers by measuring the shapes. Are they surprised by what they found out?
The line with the outward-facing arrows seems shorter than the other line.
The circle surrounded by smaller circles seems bigger than the one surrounded by larger circles. The light-coloured circle looks bigger than the dark-coloured circle, and the line that seems to pass straight through the rectangle is the one that doesn't.
Sophie Duncan is project manager for science at the BBC www.bbc.co.uksn