Watermelons require 80-90 days from seed sowing to grow a full-size watermelon. Some smaller-sized watermelons (like Sugar Baby) can reach maturity in closer to 70 days.
Some seeds can be planted directly in your garden (aka: direct sow). Other plants, however, are more often started inside, either because they cannot handle the cold conditions of the early growing season or because they need a longer season to grow that they need to start growing while there is still snow on the ground (or both).
Lettuce is a staple in most households. It's cheap to buy in the grocery store, but even cheaper to grow at home. Lettuce that is grown at home is not only fresher when you use it but it has much better flavor than the bagged varieties found in your grocery store.