Short answer for a 5-year-old: Glass mostly comes from sand. When sand and a couple of other things get very, very hot, they melt into gooey (liquid) glass. People then shape that goo into bottles and jars and let it cool.
Step-by-step:
- Start with sand (the main thing).
- Add a little soda ash and limestone (helpers to make good glass).
- Heat them up until they melt into thick, gooey liquid glass.
- Shape the goo into bottles or jars and let it cool so it becomes hard glass.
Seven simple rephrasings of the question:
- What do people heat until it turns into glass for bottles and jars?
- What melts into gooey glass to make a bottle or a jar?
- What do glassmakers melt to make bottles and jars?
- What thing turns into liquid glass when it gets very hot to make bottles and jars?
- What do you melt to make a glass bottle or jar?
- What stuff becomes liquid glass for making bottles and jars?
- What do you start with that melts into glass for bottles and jars?
If you want, I can say these questions using a toy or picture example so it's even easier to understand.