In October 2012, Microsoft released a new version of Microsoft Office Mobile for Windows Phone 8 and Windows Phone 7.8. This was followed up by the release of Office Mobile for iPhone on 14 June 2013, and Office Mobile for Android phones on 31 July 2013.
That's an easy number to work with, so let's use it. 20 drops/milliliter = 20,000 drops/liter =1,000,000 drops / 50 liters. That means one part per million is the same as one drop of a substance in a million drops, or 50 liters, of water.
A milliliter (that is milli and liter put together) is a very small amount of liquid. Here is a milliliter of milk in a teaspoon. It doesn't even fill the teaspoon! Tom says if you collect about 20 drops of water, you will have 1 milliliter: