Each of the bottles mentioned in this question contains 50 ml of liquid. The liquid in any bottle can be 100% pure content (P) or can have a certain amount of impurity (I). Visually, it is not possible to distinguish between P and I. A testing device detects impurity if the percentage of impurity in the content tested is 10% or more.
For example, suppose bottle 1 contains only P, and bottle 2 contains 80% P and 20% I. If content from bottle 1 is tested, it will be found to contain only P. If content from bottle 2 is tested, the test will reveal some amount of I. If 10 ml of content from bottle 1 is mixed with 20 ml of content from bottle 2, the test will show that the mixture has impurity, and hence we can conclude that at least one of the two bottles has I. However, if 10 ml of content from bottle 1 is mixed with 5 ml of content from bottle 2, the test will not detect any impurity in the resultant mixture.