So much has changed in the life of teenagers and young adults today. Technology, among other things, has changed so rapidly that an average childhood today looks nothing like it did 20 years ago. Twenty years ago, iPhones didn't exist. Now, 3-year-olds are playing on iPads. For us millennials, seeing staples of our childhood becoming things of the past brings more that just a little nostalgia. As the hashtag #todayskidswillneverknow trends on Twitter, let's take a look at some things that today's kids will never know.

1. Cassette Tapes

Today's kids will never know a time when music could not be "shuffled," or when you couldn't purchase a single song by itself. Today "cassette" is a word that has never entered most children's vocabularies, and they certainly don't know the struggle of having to wind the cassette film with a pencil when it became unraveled.

2. CDs and Boomboxes

Those of us born before the invention of iTunes in 2001 remember a different, more physical version of a "music library," that was best played on another ancient device, the boombox.

3. Hit Clips

To wrap up the music devices of the past, we can't forget HitClips. One-minute clips of a song never sounded so good.

4. Club Libby Lu

Unfortunately (or not), young girls today will never spend hours contemplating how to convince their parents to take them to Libby Lu. To many of us, this pink-infused retailer that offered the opportunity to be transformed into princesses and rockstars through make-up and dress-up was a dream land.

5. VHS

Kids today will never understand the frustration of waiting for a VHS tape to rewind so the next viewer could start from the beginning.

6. Crimped Hair

I know it may sound crazy, but there used to be a hair style that wasn't straight, wasn't curls, but was something a little in between.

7. Computer games

Today's kids are used to the speed and efficiency of modern Internet. However, back when the Internet had to be "dialed up," children used the computer for games. Everyone had their own collection of CD Roms, and of course, we adored Microsoft's "Paint."

8. Slap Bracelets

There was a time when slap bracelets ruled the fashion scene. Today's kids will never know the thrill (and sometimes pain) of a ruler-shaped item suddenly becoming a wearable bracelet.

9. Old TV Shows

Sadness creeps in when we realize that today's kids will never know some of TV's greatest. From "Lizzie McGuire," to "The Amanda Show," to "Blue's Clues," current children don't know what they've missed.

10. Cell Phones...before they were smart

Back before iPhones were a common sight, all we really wanted as a Motorola Razr. Never heard of that, kids? Then you probably also don't remember ABC texting, or how exciting it was when your phone finally had T9 texting.

11. Pre-breakdown Celebs

Teenagers and young adults today are lucky. We experienced celebrities like Britney Spears, Amanda Bynes and Lindsey Lohan in their prime, before all the craziness happened. But boy do we remember the meltdowns.

12. Cameras...with film

Printed photos are slowly becoming a thing of the past. Children now-a-days aren't familiar with getting pictures "developed," nor do they remember a time when a Polaroid camera wasn't a vintage collectible.

13. Tamagotchis

Throwback to our first experience with "parenthood." Today's kids will never experience the tough consequences of forgetting to feed this digital pal.

14. Legends

Unfortunately, today's kids were born too late to experience some of the noteworthy personalities of our time. From Michael Jackson, to Steve Irwin, to Robin Williams, the children of today really missed out on these inspirational individuals.

This list compiles just a handful of the noteworthy pieces of our childhood. As time passes and things change, we will just have to hang on to our memories.