Two cans of soda can be exactly the same size yet have very different densities. Density is mass divided by volume, so if one can packs more stuff into that same space, it's denser. You weigh sealed cans of regular and diet soda, then measure their volume by water displacement to calculate each can's density. Regular soda is denser than water and sinks. Diet soda is less dense and floats.

An object sinks when it is denser than the liquid around it. Raisins are denser than soda water, so they sink at first. But carbon dioxide — the gas that makes soda fizzy — collects on each raisin's bumpy, wrinkled surface. As enough tiny bubbles cling to the ridges, the raisin-plus-bubbles combination becomes less dense than the water and floats to the top. There, the bubbles pop and release their gas. Without that lift, the raisin sinks again, and the cycle repeats.

Density decides whether something floats or sinks. A raisin is denser than the liquid, so it sinks at first. Then tiny gas bubbles form on its wrinkled surface and cling there, making the raisin less dense overall. The bubbles act like tiny floats, carrying it upward. Once it reaches the top, the bubbles pop and the raisin becomes dense again — it sinks, and the cycle repeats.

Temperature changes how much stuff is packed into a given volume of seawater. When you heat seawater, some water escapes as vapor. The remaining salts and minerals are now concentrated in less liquid — so the specific gravity (density compared to pure water) rises. You heat ten beakers of seawater to different temperatures, from 20 to 65 degrees Celsius, and measure the water level and specific gravity after 24 hours. The warmer beakers lose more water to evaporation, and both pH and specific gravity rise as the temperature goes up.

Ice is less dense than oil, so ice cubes float at the top of a jar of oil. As they melt, the colored water becomes liquid. Liquid water is denser than oil, so it sinks — swirling downward in streams through the oil below.

Gas bubbles can change how dense something is. Water is normally denser than oil and sinks to the bottom. When an Alka-Seltzer tablet reacts with water, it releases carbon dioxide gas. That gas attaches to drops of colored water and makes them lighter. The drops float up through the oil. At the surface the gas escapes and the drops sink back down.