Stephen put on his hat and scarf. It was a cold day.

These two sentences can be joined using the word because.

Stephen put on his hat and scarf because it was a cold day.

We can also use the word as to join the sentences.

Stephen put on his hat and scarf as it was a cold day.

We can also join them using the word so, but we have to swap them round to do this.

It was a cold day so Stephen put on his hat and scarf.

Our writing is more interesting if we use different connecting words to join sentences.