NRICH: Perfectly Square
NRICH (University of Cambridge)

The sums of the squares of three related numbers is also a perfect square – can you explain why?

NRICH: What's Possible?
NRICH (University of Cambridge)

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

NRICH: Plus Minus
NRICH (University of Cambridge)

Can you explain the surprising results Jo found when she calculated the difference between square numbers?

NRICH: 2-digit Square
NRICH (University of Cambridge)

A 2-Digit number is squared. When this 2-digit number is reversed and squared, the difference between the squares is also a square. What is the 2-digit number?

NRICH: Picturing Square Numbers
NRICH (University of Cambridge)

Square numbers can be represented as the sum of consecutive odd numbers. What is the sum of 1 + 3 + ….. + 149 + 151 + 153?

Illuminations: Clock Squares
National Council of Teachers of Mathematics

What is the first time after midnight that the number you see (with the colon removed) is also a perfect square?