We had to learn these off in school when I was young and I remember at the age of 7 years or so, the whole class chanting "2 pints, 1 quart; 4 quarts, 1 gallon. After that, it's a little hazy but I think it was 2 gallons, 1 peck; 4 pecks 1 bushel. (This was in the UK.)
It was a bit hard to get the concept of volume and weights at that age because I remember being asked the question, "Which is heavier, a pound of feathers or a pound of lead?" And it was hard to get my head around the idea that they both weighed the same but the VOLUME of feathers would have been much greater.
Because the bushel is also a measure of dry volume, you have to take into account the lead and feathers example, which means that a VOLUME measure of lead and feathers would weigh considerably different from one another. Because of this, there are now generally accepted weights for different commodities sold in volume format, such as a gallon or a bushel, however, this is not necessarily the same in every state. I have seen a weight of 40 lbs, 48 lbs and 50 lbs quoted in different documents, for apples for Georgia and Alabama and have so far been unable to find a federal guideline with a definitive volume / weight for apples, although some documents say they follow federal guidelines but any links do not produce an official document.