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If you first got the wrong answer and then came here, don't worry about it! Most people think a milk-and-water mixture is homogeneous. Only on looking at it under a microscope do you realise that it is actually heterogeneous.
Such mixtures are called colloids. In colloids, the particles are too small to be seen by the naked eye. So, even if it appears as though a mixture is homogeneous, under a microscope, you can see that it is not.

Also, recall that if you left a suspension undisturbed for some time, the two different materials separated. If you leave a colloid undisturbed, the two different materials do not separate.