( Tanning ( ( occurs ( when ( ( ultraviolet radiation ) ( is ( ( ( absorbed ( by ( the skin ) ) ) , ) ( causing ( ( an increase ) ( in ( ( the ( ( activity and ) number ) ) ( of ( ( melanocytes , ) ( ( the cells ) ( that ( produce ( the ( pigment melanin ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Tanning)) (VP (VBZ occurs) (SBAR (WHADVP (WRB when)) (S (NP (JJ ultraviolet) (NN radiation)) (VP (VBZ is) (VP (VBN absorbed) (PP (IN by) (NP (DT the) (NN skin))) (, ,) (S (VP (VBG causing) (NP (NP (DT an) (NN increase)) (PP (IN in) (NP (NP (DT the) (NN activity) (CC and) (NN number)) (PP (IN of) (NP (NP (NNS melanocytes)) (, ,) (NP (NP (DT the) (NNS cells)) (SBAR (WHNP (WDT that)) (S (VP (VBP produce) (NP (DT the) (NN pigment) (NN melanin)))))))))))))))))) (. .)))
Tanning occurs when ultraviolet radiation is absorbed by the skin, causing an increase in the activity and number of melanocytes, the cells that produce the pigment melanin.
(ROOT (S (NP (NP (NNP Exposure)) (PP (TO to) (NP (JJ ultraviolet) (NN radiation)))) (VP (MD can) (VP (VP (VB increase) (NP (NP (DT the) (NN amount)) (PP (IN of) (NP (NN pigment)))) (PP (IN in) (NP (DT the) (NN skin)))) (CC and) (VP (VB make) (S (NP (PRP it)) (VP (VB appear) (NP (JJR darker))))))) (. .)))