You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
Bern

Context:
The city was originally built on a hilly peninsula surrounded by the River Aare, but outgrew natural boundaries by the 19th century. A number of bridges have been built to allow the city to expand beyond the Aare.

Question:
What century did Bern outgrow the peninsula?

19th century