HP wants to get Britain smiling in a world-record attempt to raise cash for leading children's charity, NCH (formerly called National Children's Homes).
To take part, all you need to do is send HP a picture of your own - and your friend's - smile.
UK shoppers at the largest shopping centres can expect to find HP's special "Smile Squads" deployed on a mission to persuade the public to smile for the camera. HP will give NCH 25 pence for each smile it receives. UK rugby star Martin Johnson is at the forefront of the campaign.
HP wants to get at least 35,000 smiles to achieve a new Guiness World Record, and all the smiles will be displayed at a London art gallery in August in what will be the world's largest photographic show.
NCH chief executive Clare Tickell said: "Smiling is a great way to make you feel good, at NCH we always focus on the positive which is why we felt this was a great project to be involved with. NCH runs more than 500 projects for children and young people, and their families, throughout the UK supporting over 140,000 people. The money raised will help NCH to give more children that we work with a future worth smiling about."
HP vice president Ian Whittaker said: "This is a chance for everyone to do their bit for a very worthy charity and be part of history. Imagine being part of the biggest photographic exhibition in the world and seeing your smile amongst the thousands of others donated. We need the British public to really get behind this record attempt, donate their smile and help us to help NCH."