Concentration of a substance in a solution
Using a FULLY MIXED sugar solution (in a closed container to prevent evaporation) as an example, if you could measure out successive 1-ml samples with perfect accuracy, and could count the sugar molecules in each sample with perfect accuracy, you would expect to find that the number of sugar molecules varies and follows a normal distribution with a mean value corresponding to the "nominal" concentration of the solution. 

You can observe this phenomenon experimentally in the case of standard solutions containing a given DNA sequence. When you create a series of increasingly diluted samples and then analyze each dilution multiple times using a PCR assay designed to detect that sequence (modern PCR assays can literally detect one molecule of DNA!), you begin to observe the expected proportion of negative sample results as you approach a dilution factor corresponding to one copy of DNA per sample. That is because when you repeatedly sample your solution containing a nominal concentration of one DNA copy per sample, some physical samples will have zero copies, some will have one (highest probability), some will have two, etc. with decreasing probability as you go up.