封筒【ふうとう】
envelope
水筒【すいとう】
canteen, flask, water bottle, thermos
筒井【つつい】
round well
発煙筒【はつえんとう】
smoke bomb, smoke candle
円筒【えんとう】
cylinder
気筒【きとう】
cylinder
井筒【いづつ】
well curb, well kerb, family crest design based on a square wooden well curb
筒抜け【つつぬけ】
being overheard, being leaked (e.g. secrets), going in one ear and out the other