This article for teachers discusses examples of problems in which there is no obvious method but in which children can be encouraged to think deeply about the context and extend their ability to. . . .

Drawing a triangle is not always as easy as you might think!

Can you find the values at the vertices when you know the values on the edges of these multiplication arithmagons?

Can you find a rule which relates triangular numbers to square numbers?

Explore the relationship between quadratic functions and their graphs.

Can you find a rule which connects consecutive triangular numbers?

Show that all pentagonal numbers are one third of a triangular number.

The points P, Q, R and S are the midpoints of the edges of a convex quadrilateral. What do you notice about the quadrilateral PQRS as the convex quadrilateral changes?

The points P, Q, R and S are the midpoints of the edges of a non-convex quadrilateral.What do you notice about the quadrilateral PQRS and its area?

Many numbers can be expressed as the difference of two perfect squares. What do you notice about the numbers you CANNOT make?

Think of a two digit number, reverse the digits, and add the numbers together. Something special happens...

Do you notice anything about the solutions when you add and/or subtract consecutive negative numbers?

Alison has created two mappings. Can you figure out what they do? What questions do they prompt you to ask?

Problem solving is at the heart of the NRICH site. All the problems give learners opportunities to learn, develop or use mathematical concepts and skills. Read here for more information.

Investigate sequences given by $a_n = \frac{1+a_{n-1}}{a_{n-2}}$ for different choices of the first two terms. Make a conjecture about the behaviour of these sequences. Can you prove your conjecture?

In 15 years' time my age will be the square of my age 15 years ago. Can you work out my age, and when I had other special birthdays?

Show that for any triangle it is always possible to construct 3 touching circles with centres at the vertices. Is it possible to construct touching circles centred at the vertices of any polygon?

Take any whole number between 1 and 999, add the squares of the digits to get a new number. Make some conjectures about what happens in general.

Have a go at creating these images based on circles. What do you notice about the areas of the different sections?

Can you find some Pythagorean Triples where the two smaller numbers differ by 1?

A serious but easily readable discussion of proof in mathematics with some amusing stories and some interesting examples.

Here is a machine with four coloured lights. Can you make two lights switch on at once? Three lights? All four lights?

Use the computer to model an epidemic. Try out public health policies to control the spread of the epidemic, to minimise the number of sick days and deaths.

Find out about Magic Squares in this article written for students. Why are they magic?!

Take any pair of two digit numbers x=ab and y=cd where, without loss of generality, ab > cd . Form two 4 digit numbers r=abcd and s=cdab and calculate: {r^2 - s^2} /{x^2 - y^2}.

Janine noticed, while studying some cube numbers, that if you take three consecutive whole numbers and multiply them together and then add the middle number of the three, you get the middle number. . . .

Helen made the conjecture that "every multiple of six has more factors than the two numbers either side of it". Is this conjecture true?

Charlie has created a mapping. Can you figure out what it does? What questions does it prompt you to ask?

Pick a square within a multiplication square and add the numbers on each diagonal. What do you notice?

Explore the relationship between simple linear functions and their graphs.