My school also uses Turnitin. It's a program that literally checks for similarities of every known piece of writing available, and then returns it's check with a report and a percentage that will state how much you've plagiarised.
Generally, teachers can google a sentence and find the exact source of information also. And also, when you have 20ish students doing a similar assignment, and use the internet as a source, similar sentences also appear in each students assignment; then teachers detect the plagiarism occurring.
This site is fairly well known by the teaching staff at my school; they even recommend it for help and as such. So, depending on your school it might be vary on whether information sourced from here will be picked up on.
And on the note, 2 paragraphs is quite an extreme amount to plagiarise. It really cannot be that hard to at least reword it, and then put some of your own ideas into the paragraphs to at least give it a sense that it's your own piece of writing.