1: Shrink Rays
Yes they all seem like fun but what do they do? They decrease something's physical size but there is no mention of one rather important factor. Mass. The mass or weight would remain constant, the object would just become incredibly dense. Think of it this way, if you put a car in a crusher that 2 ton metal construct will weight two tons regardless of if it's the original car shape of the much smaller cube.
So let's say you shrank the Moon and bought it to Earth. It might be small but that small lump of rock would have a mass of around 7.35x10^22 Kg. Couple in Earth's gravity as weight is mass x gravity and you're left with a very heavy mall of rock that would be exerting it's own gravity on you anyway and causing all kinds of interesting problems.
Ok so how about shrinking people? Well same problem applies, if you weighed 50Kg before shrinking, you'll weigh that after but you'll exert more pressure as your weight will be concentrated over a smaller area. What does this mean? If you go walking along the beach be prepared just to sink in the sand and wait for the inevitable. There's always the problem of nutrition too, remember everything about you is so much smaller than it once was and this includes all those lovely enzymes you need for life. Now let's say you were thirty so you had a drink, seems fair doesn't it? But what if all those non shrunken water molecules are now simply too big to do their osmotic thing and dehydrate you? What is the food you eat is simply too big for your enzymes to work on? Well they your days are numbered.
Take home message: If you think something should be smaller, use less materials when manufacturing it or simply crush it. Don't shrink large objects or living things.
2: Enlargement rays
Is the world getting you down? Do you just feel the need to go on a Godzilla style rampage to blow off some steam? Did you just buy or invent and enlargement ray to do that? Well I suggest you take a minute to look up.
As with shrinking, making yourself bigger will not increase your mass. This means you'll still be 80Kg but a much bigger target. You wouldn't be crushing cars underfoot, you're more likely to trip over them. Can you punch through concrete walls now? Well don't suddenly expect to unless you can increase the force behind your blows your fists will weigh the same and due to their increased size will exert less pressure over that wall you hate so much.
Also think about food, roasting a cow might seem like a good idea but remember, your enzymes are now way too big for their substrates. They simply won't work. Think you can always shrink yourself to eat? Well remember oxygen? Do you think your oversized haemoglobin will bind that teeny tiny little molecule properly now? Think the electron transfer chain will happily accept it? Well let me know how it turns out because the risk of hypoxia doesn't seem worth it.
3: Explosions in space
Everyone loves them in films, they look cool, sound better with all that bass and make it all worthwhile. What a shame you would never hear them, all those cool laser firing effects. Why is that? Well what is sound? It's vibrations which hit our eardrum which are they amplified and passed onto the cochlea where fluid moving hairs on cell surfaces creates nervous impulses.
What's the point of this? Well space generally lacks anything dense enough to transmit vibrations through. If it did it would suggest that for us to hear everything with such clarity space was filled with a gas with the same density as our atmosphere. So why do we perpetuate this lie? Well would the Death Star explosion have been as impressive if it was just a silent light show? If you say no then it's your fault this lie lives with us to this day.
4: The scientific MacGuffin.
This can take many forms but it is always a plot device explained by science which is invaluable later on. Let's take one example shall we? The use of EDTA to explode vampires.
EDTA or ethylenediaminetetraacetic acid is commonly used as an anticoagulant. Often as the sodium or potassium salt which is a lovely white power and dissolves into a colourless solution (not a blue one). Ok but what does EDTA do? It sequesters divalent ions that is ions which carry a 2+ charge like calcium. Now calcium is good at making our teeth and bones strong, it's also required to form clots when you start bleeding. When EDTA comes along it grabs all those calcium ions and keeps them to itself stopping clots forming.
Now this particular franchise likened vampirism to a virus, so let's ask why removing a small amount of calcium would make a vampire explode? Does this virus become highly volatile with very minute changes in circulating calcium or does it trigger instant proliferation of blood cells causing things to explode if just one of those precious calcium ions is removed? I would say this is highly unlikely. So why use it? Well people have heard of EDTA although they probably don't know what it actually does so this knowledge was abused to drive the plot forward. This is the important thing, this entertainment relies on ignorance to be effective.
So there you are, 4 little examples of how movies are used to make money off of us while perpetuating fallacy and relying on the ignorance of the public. When it comes down to it the writers are basically saying "because it's scientific we don't have to explain it because you won't understand anyway" so take you time to think. Was that movie really cool or was it insulting your intelligence while taking your money?