# Is the Glass Half Full

You have a glass of water that looks about half full. How can you tell, only using the glass of water itself, if the glass is half full or not?

The glass is a right cylinder.

### 5 guesses to Is the Glass Half Full

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• Frank

grab the glass with your finger marking where the level of the water currently is. Then, place your hand over the top of the glass and flip it upside down. Check if the water level is the same or above or below the point where your finger is.

• Jim

Tilt the glass until the water in the glass just touches the rim. If the level of the water toward the bottom touches where the bottom and side make a right angle the glass is half full.

• Crabman

Jim is telling part of the story, but not all of it. Tipping the glass to where the water just touches the rim, if the back of the water does not reach the top side, it’s less than half full, if it just reaches it, it’s exactly 50% full, if it more than just touches, it’s more than half full.

Option 2: drink the water, or pour it out. It’s now definitely less than half full.

Option 3: I’m trying to figure out if we can somehow know when the glass it tipped exactly 45 degrees. If you can’t tip it 45 degrees without spilling it’s more than 1/2 full, exactly 45 degrees it will be exactly 1/2 full, if you can tip more that 45 degrees without spilling it’s less than 1/2 full.

Hmmm… if you move the glass exactly the distance of its height from a wall that is exactly perpendicular, then tipping it toward the wall it will be 45 degrees when it touches the wall.

Still, this requires a wall and it’s hard to measure its height from the wall with water in it. Maybe a level light source and it’s shadow.

Tipping until the water touches it’s lip us simplest, I may have taken a while to get there without Jim’s post.

• Jim you are today’s winner!
Tip the glass of water until the water reaches the rim of the glass and if the water lines up perfectly with the bottom rim of the glass, it is half full.

• Peebs

All the way full, part water, and part air.