Based on the information present in the given passage, you need to write a correct answer to the given question. A correct answer is one that correctly and completely answers the question. Do not write answers that address the question partially or incorrectly. The correct answer must be contained in the given paragraph.

Paragraph- Sent 1: Oh no!
Sent 2: Your younger sister just mixed up all of your LEGO parts.
Sent 3: Now you have to put them all back into the original categories.
Sent 4: How will you do this?
Sent 5: You sort them by size and shape until they are each back into their specific place in the tray.
Sent 6: What do you think you could have called the mess your younger sister created?
Sent 7: Thats right, it is a mixture.
Sent 8: Fortunately, it was a physical change and she just made a simple mixture.
Sent 9: You are able to separate them back into order.
Sent 10: Its a good thing she did not toss them into the fireplace.
Sent 11: That would have caused a chemical change as they all melted together.
Sent 12: If your LEGOs were melted, you would be out of luck for building that next big project. 
Question: Why do you have to sort your LEGOs and put them in the tray?.
Your sister mixed them together.