What does "stand the test of time" mean?

What does "coils" mean?

"Coils" means something that is twisted and rounded into a spiral or ring shape. Baloo uses "coils" as a metaphor in this video. When Baloo calls is "the coils of death" he is saying that Mowgli was dying and he would have died if Baloo did not save him.

What does "keep the change" mean? In what circumstances could I use it?

Generally, when people pay for something with cash (bills and coins), they will not have the exact amount (which is called "exact change") and will thus pay with a larger amount than the set price. Thus, you will usually receive something called "change" back. When you tell someone "keep the change," it means that you don't want the leftover amount. People who use this expression might say it for a number of reasons: they dislike carrying around coins, which are bulky and heavy; they don't think the amount of change is worth taking as it may be worth too little; or they mean for the cashier/employee helping them to keep it as a small tip (this depends on the type of store and service). Ex: A: Your change is ten cents, sir. B: It's alright, keep the change. Ex: Keep the change. Thanks for your help today.

is "stimulant" a countable noun? Why is an article used here?

That's right! It is a countable noun. I think when spoken about generally, the plural term "stimulants" is used, but here they're talking about one kind of stimulant being carried through, so that's why "a" is used. Ex: They have a dish called lobster lasagne at the restaurant, it's so good. Ex: Caffeine is a great stimulant to have in small amounts!

Is there any words other than "pay" or "give" to use with "attention"?

"Give" and "pay" are the most common verbs used before "attention". However, you can also use the verb "show". Ex: I show a lot of attention to my dog.

Is “water” a verb here?

Yes. Water is used as a verb in this sentence. To "water" means to supply water to plant, or soak with water. Ex: You need to water rosemary every once a week. Ex: The gardener waters the garden twice a day. The verb form of "water" can also be used to supply water to animals for drinking. Ex: I need to water the cows.