This is a pretty quick and simple instructable, so let's dive right into it.
You will need:
-A glass bottle with a screw on top, such as IBC rootbeer.
-A plastic 2L soda bottle
-A knife and/or a sharp pair of scissors (If you are a young Do-It-Yourselfer, ask a parent or someone else who is responsible with sharp objects to do the cutting for you)
Optional: Aluminum Foil
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-Knives and scissors are sharp! Do not cut toward self. Do not lick blades. Do not ingest plastic trimmings.
-Hot beverages are hot. Wait till it is not scalding before drinking it. Do not spill hot drinks on your legs/arms/torso/crotch/etc.
-Use common sense! If some part of the back of your brain is saying "That's a bad idea," it probably is! Don't do it!
Step 1: Wash the Glass Bottle
Wash out the glass bottle with hot soapy water.
Step 2: Off With Its Head!
You want a hole big enough for the neck of the glass bottle to just fit through.
Step 3: Short Division
You want the bottom half slightly bigger than the top half.
Step 4: Stuff It
Put the glass bottle in the bottom half of the plastic one. Stuff around the outside of the glass until it's snug in place.
*Also, you may want to wrap the glass bottle in aluminum foil before you wrap it in the paper towel. This will serve to reflect some of the heat energy back into the bottle. However, I would limit the wrapping to the thick part of the bottle, only, because Aluminum has a low heat capacity (it's a thermal conductor) and if it comes in contact with the cold air, it will only serve to channel heat out of the thermos.
Thanks to rimar2000 for this tip!
Step 5: Nip/Tuck
Make an inch long vertical cut in the bottom half in a similar manner.
Step 6: All Together Now
Step 7: Fill 'er Up
in a Pyrex measuring cup or something with a spout, so you don't spill your precious tea everywhere.
Cap it, take it with you, and enjoy the occasional puzzled looks. However, once they realize how clever you are, those looks will change to awe and jealously.