What can i put im a 2 liter bottle to make the temperature rise inside it?

Question

like school safe objects i can cut open the bottle and then tape it back

Answer 1

the safest( and most boring) thing i can think of is tape black paper around the outside of the bottle, and then leave it in the sun.(the heat is absorbed by the paper, which the heats the air internally). ANother one which is a little more exciting is to add 1 teaspoom of drain cleaner crystals to a bottle of water (around 100mL). Add in 3 scrunched up balls of foil(ensure they are really scrunched up). The drain cleaner attacks the foil, releasing heat.(be careful, gas is released which can make the bottle explode if you put the lid on too soon, or if you go overboard with the ingredients, melt the bottle and ignite the gas. ensure you follow the quanities given)

Answer 2

By far the simplest is hot water.