I do this with a pack of number cards, marked from 1 to 34 (enough for my largest classes that way). Give each pupil a card, give them a minute and they have to tell you something interesting about that number. I let them decide what's interesting - it can be that it's someone's birthdate or the number of pets they have, or it can be a mathematical fact.
So here's seven interesting facts about the number seven:
1. It's the lowest natural (counting) number that can't be written as the sum of three square numbers. If you want to write 7 as a sum of squares, you have to use four: 1² + 1² + 1² + 2². Incidentally, Lagrange's Four Square Theorem is a great way to practice square calculations without doing reams of textbook questions.
2. It's a lucky prime, which, as far as I can fathom, is a completely useless but quite interesting property.
3. There are seven heavenly virtues and seven deadly sins. The deadly sins give us Se7en, horrible to type but the title of an excellent film starring Brad Pitt and one of my all-time faves, Kevin Spacey - watch it, it's good.
4. The number seven is considered mystical or magical, and is religiously and culturally significant. In Christianity, the world was made in seven days. There were seven wonders of the Ancient World, historically it was believed there were only seven seas, and there are seven continents that make up the world. The Pythagoreans loved the number 7 because it combined the numbers 3 and 4, which, in representing the triangle and square, were hugely significant to their ideas about numerology.
5. The number seven is also considered lucky - it's the jackpot on most slot machines. It's theorised that this is to do with both its links to the mystical, and also because seven is the most likely number to be rolled on a pair of six-sided dice (there's 6/36 = 1/6 chance).
6. There's quite an interesting theory that short term and working memory can successfully hold and work with around seven items at the same time - see Miller's Law and this article from Psychology Today.
7. When people are asked to think of a single digit, the most frequently picked is 7. According to Alex Bellos (Alex Through the Looking Glass), this is because all the other numbers (evens, multiples of 3, 0, 1 and 5) all feel rather unspontaneous.
(Image credit: Botanischer Volkspark Blankenfelde-Pankow by Sebastian Wallroth - Own work, CC BY 3.0, commons.wikimedia.org/w/index....)