Why is Friday the 13th unlucky?

This is from the BBC website, and I think it makes perfect sense out a superstitious day...

Karen

Now some things you hold on to - and some you just let go
Seems like the ones that you can't have
Are the ones that you want most

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Friday the 13th has traditionally been when the superstitious take extra care and double check everything they do.

Now experts at the Glasgow Science Centre are looking into the global phenomenon to determine if there is any science behind the superstitions.

GSC said it is a mix of superstitions - Friday being the unluckiest day of the week and the number 13.

They also found superstitions around opening an umbrella indoors dating back to the Ancient Egyptians.

Umbrellas were used by Egyptians to protect themselves from the sun, and opening an umbrella indoors was seen as an insult to Ra the Sun God who it was feared would punish you with terrible luck.

GSC's senior science co-ordinator Andy Laing looked into how, scientifically, umbrellas may actually be unlucky.

He said: "Storing a wet or damp umbrella in an area within your home which has poor or no ventilation may cause it to spontaneously combust.

"There is the theoretical risk that some umbrella material can heat up so much during the decay process that it would burst in to flames.

"I don't know about opening an umbrella inside, but your umbrella spontaneously combusting would definitely be bad luck."

Mr Laing said another widely held superstition is that walking under a ladder is unlucky.

It is said to stem from the Christian belief the triangle was the symbol of the Holy Trinity and therefore, by walking through the triangle - made by the ladder, wall and ground - you were being disrespectful to God.

However, Mr Laing said there is a more practical explanation for it being unlucky to walk through ladders: "Imagine how unlucky you would be to walk under a ladder and the window cleaner's bucket tips over you? Maybe not unlucky but being a bit silly and definitely soggy."