The number of blocked sewers across the UK is higher than ever at this time of year.
Ricky went underground in a sewer to find out what's causing the problem...
It's my first day back in the Newsround office and I'm ready to report on the stories that matter to you.
So what's my first assignment of 2009? Looking at the latest stories in the music world or how about checking out the latest trends at your school?
It's hot, loud and very very smelly inside the sewers
Nope, instead the Newsround bosses sent me down a sewer in London to investigate a problem that loads of you never get to see.
Deep under the streets, sewers are overflowing with oil, fat and grease.
It all comes from cooked fat that sometimes gets washed down the kitchen sink or poured down the drain.
When it reaches the sewers, the hot fat cools and turns into a hard substance, a bit like polystyrene. And that's when the problem really begins.
Over time the fat clogs up the underground network of pipes and causes blockages. In the worst case scenario, sewage and fat can overflow into the streets!
The cooked fat goes really hard when it cools in the sewers
I went to see all the fat that ends up in the sewers for myself. Apart from the really bad smell of toilet waste the sewers were pretty fascinating.
Thames Water is a company that looks after the sewers and water-pipes in the South East of England. They took me down the drains to see all the fat for myself.
I had to take part in a safety briefing because sewers can be a dangerous place. I also had to wear special protective clothing.
Hard to hear
It was really warm inside the sewer, around 22 degrees, while back on the street it was -1 degree.
The sound of water was really loud. It was hard to hear my cameraman and the rest of the people I was interviewing.
Ricky's looking a bit grubby after his sewer visit
A team of experts told me that this is the time of year when even more fat clogs up sewers across the country.
Thames Water said more than 500 tonnes of fat ended up in the sewers over the festive period.
That's because more people are cooking things like Christmas turkey and then throwing away the fat and oil down the kitchen drain.
Their advice is to reuse the oil and fat by making gravy. You can also make bird feed from it once the fat has cooled - all you have to do is add nuts to it.
But they strongly recommend that adults dispose of the fat in appropriate ways by throwing it away in silver foil and then putting it in the bin.