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I suspect I'll have several of these queries in the near future.... Sweajnr (almost 6) has been given a big tub of Lego by an elderly neighbor because "her son doesn't want it anymore". This makes me sad as all my Lego is still with my parents for when the grandchildren visit. Anyway such is life and we now have a lot of Lego but no instruction booklets to go with it. Whilst random building is fun it's also fun to make kits especially if you've never made it.
Below is a piece that sweajnr has instantly been attracted to because it makes noise and flashes. So does anybody know what kit / era this might have come from and where the instructions for building the original piece might be found online?

Wow. That's fantastic!! (and scary)
Thanks!!
STW never fails to amaze me! Good work Kneebiscuit
Somewhere on the piece there will be a 4 or 5 digit number... you can then use that number to look up the piece on bricklink or other site...
You might need to look quite hard sometimes though...
Well the previous effort was so successful here's another one (not one piece this time and despite lots of looking I've not found any of the ID's funkynick talks about....,

Look around bricklink. Those jet engine pods should be easy enough to find. Follow the links to kits found in and if your in luck you will recognise the shapes.
E.g. The engine in grey; https://www.bricklink.com/catalogItemIn.asp?P=4868b&colorID=86&in=A
Second set; https://www.bricklink.com/v2/catalog/catalogitem.page?S=75087-1
Search for the instructions for 75087-1 and you get this; http://lego.brickinstructions.com/en/lego_instructions/set/75087/Anaki n's_Custom_Jedi_Starfighter or a llok on the lego site gives the full PDF ( https://www.lego.com/biassets/bi/6119557.pdf). Step 32 should look familiar :D.
Rebrickable is also a good place to find instructions; https://rebrickable.com/instructions/