The first time I found it, it was by the side of the television and I assumed it was one of the kittens – even though there was rather a lot of it and they had never done anything like this before.

But then two days later another huge patch appeared behind my son's toy box.

When I picked Samuel up from school, I asked him if he knew how the wee had got there? He looked up at me with wide-eyed innocence and assured me he had no idea.

"It wasn't me mummy, I promise," he said.

"I do know who it was though."

"Oh?" I said.

"Yes. Daddy."

Of course I knew this wasn't the case. Then suddenly, Samuel started to laugh, or should I say giggle.

"Was it you, Samuel?" I asked.

"No." he said.

"Why are you laughing then?"

"Because I think it's funny," he said.

"You do realise how important it is to tell mummy, the truth, don't you?" I asked.

"Yes. If it was me, would you be cross with me?"

"No. I would be very sad though if I find out you have been lying."

"Mmmmm. But it wasn't me, honest."

"OK, Samuel. I believe you," I say.

But the problem is, I am not sure I do.

Of course, I can't prove it was him and I have absolutely no idea why he would do such a thing.

I think what has upset me the most, is the fact I always thought I would be able to tell if he was lying to me, but now I am not so sure.

He sounds and looks convincing, but if it isn't him – who the heck is it?

It isn't me (obviously) and it isn't my husband, and there is no one else living in our house – except of course the kittens, but without being too gross about it, the wee looks (and smells) human.

I cannot believe Samuel would lie to me so blatantly. If he is being dishonest, where has he learnt to do it so well?

And if it was him, how do I get him to tell the truth?

One friend of mine says when she knows her daughter is lying, she has introduced "the policeman".

"I tell her, if she doesn't tell me the truth I will phone the policeman and he will find out what happened," she said. "For some reason, this works a treat and she spills the beans almost immediately."

I am not sure if this will work on Samuel, because he is a few years older than my friend's little girl and is a bit too savvy. He would probably quite like to meet a policeman too.

Of course I will give it ago, and there is always the possibility that he is telling the truth, and I will have to eat mounds of humble pie.

I'll let you know how I get on...