Hi Londondee I agree with opalstones - if you don't feel confident to answer the question - then work with your DS to discover the answer - it shows him that you enjoy learning about things and will help him to be confident to find out more when he doesn't know the answer (later in life).
Some really useful websites:
BBC Bitesize KS1 Science: www.bbc.co.uk/bitesize/ks1/science/ - you can select the abilit level (medium/ hard/ really hard) and through playing games he can learn more about things.
There are science programmes in tv which are child friendly: Cbeebies Nina and the Neurons does a lot about how things work/ why things happen.
CBBC is for slightly older kids mainly but has a range of shows but you can see clips here: www.bbc.co.uk/cbbc/watch/by/theme/science
National Geographic has a dedicated children's website: www.ngkids.co.uk/
Khan academy has linked up with MIT students to set up little videos on various STEM topics for K (kindergarten = English Year R) to Grade 12 (=UK sixth form upper or Year 13). link here: www.khanacademy.org/partner-content/mit-k12
the Khan academy has also linked up with the British Museum here: www.khanacademy.org/partner-content/british-museum
You can also explore the Science Museum on-line: www.sciencemuseum.org.uk/online_science.aspx
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There is science programming on CBeebies (Nina and the Neurons) and CBBC (e.g. Deadly 60 for interesting/ exotic animals).
Some of the science documentaries have been absolutely fabulous for young children: Spring/ Autumn Watch, Stargazing live, Bang Goes the theory, even Cat Watch (www.bbc.co.uk/programmes/b04lcqvq) etc.... have all sorts of great information about how things work and why.
Gardeners World (BBC1) can teach your child a lot about plants. Growing his own plants - maybe something he can eat - this spring is a simple thing you can do at home. Just need a packet of seeds, a pot and some compost - it you don't have a garden.
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How stuff works: home.howstuffworks.com has all sorts of information - for example your light bulb (how do headlights in the car work) question can be partly answered here: home.howstuffworks.com/light-bulb.htm - if you buy an old fashioned bicycle light with a dynamo (do they make them any more?) - you can attach it to your child's bike and he can make the electrical current to charge his own head light. In essence this is how a car's headlight works.
and as others have suggested - wikipedia is full of information....
I think the thing to bear in mind is that at age 5 you can't really explain precisely what is going on (the physics/ maths/ chemistry/ biology/ etc... behind it in detail) but you can learn a bit yourself and generally explain key points. And you can create opportunities for understanding the process by simply helping your child grow some plants this spring/ summer.
Just be patient with yourself. If you don't know it at first - don't panic. Explain what you can.
visit libraries, museums, special events, exhibitions - which are related to your DC's interests and encourage that interest. You don't have to answer every question he has - you just have to help give him to tools to start to answer it for himself.
I can remember my DD1 asking how ice cream was made aged 5 and it was only year's later whilst watching a little historical clip about the history of ice cream on The Great British Bake Off (now aged 12) that she learned about it in much more detail in an excellent little clip narrated by Sue Perkins. Our solution at the time was to dig out an underused wedding gift and make some homemade ice cream together with a modern electrical ice cream maker, which we now do every summer with fruit we grow or collect. I certainly didn't know about the glass penny lick containers prior to ice cream cones - an English invention apparently.
And I think that's the point - there isn't just one answer and there often are many angles to learning about something - it's history, who used or had it, who built it and why, how old it is, how it works, etc.... And the more you look into something quite often the more you find out...
I suddenly feel the urge to go to Italy and research deeply into their glorious gelato.
HTH